​1. Ron Mahay Becomes a Pitcher
Ron Mahay is one of those guys who has been around forever, and for one simple and obvious reason: He throws the ball with his left arm. Mahay came up with the Red Sox in 1997, was claimed off the waiver wire by the A's (who even tried him as a starter a couple of times!), was purchased by the Marlins, signed with the Cubs for two years, then spent four years in Texas. He managed to be "the other guy" that the Braves got when they traded Elvis Andrus, Jarrod Saltalamacchia, Matt Harrison, Neftali Feliz, and um, Beau Jones for Mark Teixeira.

Despite the fact that he functioned near or often below replacement level for most of his career, at age 37, Mahay, who for his career averaged 7.4 strikeouts and 4.2 walks per nine innings while generally pitching exclusively in a role where he had the platoon advantage most of the time, signed a two-year/$8 million contract in free agency. #Royals.

As of 2012, he was still kicking around the minors, even spending some quality time with the Reds' Triple-A affiliate. But did you know that Mahay actually made his major-league debut in 1995… in center field? Mahay played five games there for the Red Sox, who suggested that Mahay, who had a good left arm, go bac into the minors and try pitching. If there is ever proof needed for the theorem that left-armed equals 10-year MLB career (minimum), it's Ron Mahay. Any despondent southpaw position player trying to re-invent himself in major-league baseball should take note. —Russell A. Carleton

2. Dennis Eckersley Converts to Relief
Dennis Eckersley was a pretty darned good starting pitcher. He won 20 games for the 1978 Red Sox one year after pitching a no-hitter for the Indians. Eckersley also had digit wins in his first six seasons as a starter and 10 of his first 12 until 1986, when he fell to 6-11 with a 4.87 ERA for the Cubs. An admitted hard-partier as a youngster, Eckersley was out of shape and seemingly on his way out of baseball at 32 when the Athletics acquired him in a five-player trade in the final days of spring training in 1987. Manager Tony La Russa and pitching coach Dave Duncan converted Eckersley into a closer and the rest, as they say, is history. Eckersley pitched for 12 more seasons, saved 390 games, and wound up being inducted into the Hall of Fame. The greatest re-invention of all might be that Eckersley continues to turn back the hands of aging in his career as a television analyst. He is now 58, sports a perpetual tan, and looks like he is 20 years younger. That makes a guy who is nine years younger than Eckersley but looks 10 years older just a tad jealous. —John Perrotto

3. Adam Jones and B.J. Upton Move to Center Field
In 2005, B. J. Upton committed 53 errors in 133 games in Triple-A, and in 2006, he committed 28 more in 84 games. He was playing shortstop in those games, and although one Durham eyewitness still remembers Upton making the greatest play he’s even seen a shortstop make, baseball is a game that abhors extremes. Upton moved to third base, tried second base, then was finally put out to the pasture of center field, where he found a place to make himself comfortable as a Tampa Bay mainstay and, finally, $75 million from the Braves.

Adam Jones, too, found life a lot easier out in center field after starting out as a minor-league shortstop who booted balls all over the place. (Both he and Upton had range factors at shortstop that rivaled Omar Vizquel’s. Ah, well…) No surprise that the Reds made the same move with Billy Hamilton, who can now utilize his single greatest tool—one of the greatest in all of sports—unburdened by his former position. How many careers have been made by dint of getting players away from shortstop and its steep demands? And how many never came to be, simply because the men who tried to undertake them were simply stationed at the wrong post, and never reassigned to one they could handle? —Adam Sobsey

4. Tim Wakefield Becomes a Knuckleballer
If you scroll through the 1988 draft, all the way to the eighth round, in between Tim Naehring and Mike Smedes, the Pittsburgh Pirates used the 200th pick to take a power-hitting first baseman out of Florida Technical College. Tim Wakefield's alma mater isn't exactly a baseball powerhouse. In fact, if you look it up online, the website advertises the school by offering all incoming freshman new iPads. Wakefield spent his first pro season in Low-A Watertown, New York hitting .189/.328/.308. That must have got the point across that he wasn’t going to make it as a hitter. The next season he stepped on the mound as a knuckleball pitcher.

As a hitter, Wakefield would likely have lasted another season or so before getting released. As a knuckleball pitcher, he shot through the Pirates system, making his major-league debut after just two seasons. He finished third in the Rookie of the Year voting in 1992, and after a bit more time in the minors, he wound up with the Red Sox, winning 200 games over the next 17 seasons for Boston.

For some players adding a new pitch or moving across the rubber can be the difference between success and failure. For Wakefield, the knuckleball was the difference between major-league baseball fame and fortune, and… dentistry? Wakefield kinda looks like a dentist… —Matthew Kory

5. Scott Feldman Modifies His Delivery, Adds a Cutter, Gets Paid
It’s common for traditional over-the-top pitchers to “drop down” sidearm once they enter professional ball, but the reverse rarely happens. Feldman did both early in his pro career. The 6-foot-7 righty initially dropped down sidearm shortly after being drafted by the Rangers in 2003. Less than two years later, he was pitching out of the big-league bullpen. But as he began to scuffle in 2007, Texas had him return to the standard three-quarters delivery, introduced him to a cutter, and later had him join the starting rotation.

Two years after making the rare switch, Feldman rode his traditional arm slot to 189 innings, 17 wins, and a 4.08 ERA in the Rangers’ rotation. Was that season the “real” Scott Feldman? Absolutely not. Feldman’s cutter was fantastic in 2009, but he hasn’t approached that success since. Did the re-invention get him paid? You bet it did. While it’s safe to say Feldman is a No. 5 starter at best these days, the unorthodox adjustment sent him from short-shelf-life middle reliever to a man who has made nearly $5 million annually since. —Jason Cole

6. Sean Doolittle Moves to the Mound
Doolittle was selected in the supplemental round of the 2007 draft, taken by the Oakland Athletics at 41st overall. A left-handed first baseman out of the University of Virginia, Doolittle was praised for his hitting skills yet had questionable power projection, creating an uphill battle for him to carve out a career as a first baseman at the highest level. The power failed to materialize in the minors, save for a half-season vacation in the California League, and recurring injuries to his knees stalled Doolittle's development, clouding his path to the big leagues.

The left-hander was a two-way player in college, with the scouting notes from suggesting that he threw “87-90 mph from the mound,” and the A's opted to reverse course with Doolittle to see if he could make an impact as a major-league pitcher. He entered the 2012 season with an assignment out of the bullpen for Stockton, tasked with surviving the very environment that had fueled his own offensive breakout a few years prior, and with five years having passed since his last extended exposure to the rubber.

An adjustment period was to be expected due to Doolittle's prolonged hiatus combined with the difficulty of transitioning to the mound, yet the southpaw skyrocketed through the minor leagues, jumping through three levels and climbing to the majors within two months. He allowed just three earned runs across 25 innings in the minors, with a K-to-walk ratio of 48-to-7, and the dominance carried over to the majors without Doolittle missing a beat. The southpaw was more vulnerable to left-handed bats during his major-league time, with a reverse platoon split that could be a fluke of small sample size or perhaps a reflection of Doolittle's lack of secondary stuff. He threw 87 percent fastballs last season, averaging 94.4 mph, and the one-pitch wonder managed a 60-to-11 strikeout-to-walk ratio across 47.3 innings in the majors.

The first week of 2013 was more of the same, and Doolittle's lightning-quick adjustment from the batter's box to the mound was one of the more intriguing subplots of the Oakland A's exhilarating run to the AL West crown last year. —Doug Thorburn

7. Mike Scott Learns the Splitter
Mike Scott had pitched six seasons in the major leagues at the conclusion of the 1984 season. In 663.2 innings of work, Scott had walked 211 hitters while striking out just 307 and had a 4.45 ERA to show for it. The 4.2 strikeouts per nine ratio was alarmingly low for someone who threw in the mid-90s but had nothing to complement his heater with. As the legend goes, Astros teammate Enos Cabell suggested Scott reach out to Roger Craig over the winter to learn the split-fingered fastball. Cabell had seen how well the pitch worked for the Tigers when Craig was their pitching coach. Scott took to the split-finger like a pig to slop and became a two-pitch pitcher with that hot fastball and a splitter that dropped off a table. That first season, Scott won 18 games and struck out 5.6 batters per nine, which was a career high for him, but the magic began in 1986, when he struck out 306 hitters in 275 innings of work, had a league-best 2.22 ERA, and won the Cy Young.

His strikeout total declined each season after that, but Scott had five consecutive seasons of 200 innings or more, and from 1985 to 1989, he went 86-49 with a 2.93 ERA and struck out 7.8 batters per nine innings. There were always whispers, nay, screams of Scott doctoring the baseball, most vociferously by the Mets in the 1986 postseason. To the surprise of absolutely nobody, Scott confirmed the accusations in an interview with MLB Network in 2011, thus joining a long list of pitchers who eventually came clean: "They can believe whatever they want to believe. Every ball that hits the ground has something on it. … I’ve thrown balls that were scuffed, but I haven’t scuffed every ball that I’ve thrown." —Jason Collette

8. Brian Bogusevic Starts Hitting
Players who convert from a position to pitcher are usually among my favorites, because it’s the weird freaky things that tell us something new about baseball. Players who convert to pitching and succeed quickly tell us something about the beautiful simplicity of pitching. Once I talked to Sean Doolittle about this, and he said the development curve for young players makes the opposite move—from pitcher to hitter—almost impossible. “The biggest thing is the timing. Taking several years off to go back to hitting would be really tough. There have been some guys who have done it, and that just blows my mind.”

One of the guys who has done it is Brian Bogusevic, who is hitting .389/.476/.389 in five games for the Cubs’ Triple-A affiliate as a 29-year-old. This is supposed to be a Lineup Card on big changes that worked, and so it might not seem like Bogusevic qualifies. But he’s got 612 big-league plate appearances and made about a million bucks as a hitter, which is a lot more than you’ve ever done, hot shot.

That’s after spending nearly four years as a pitcher exclusively in the Astros’ system. But the former first-rounder was stalled and going nowhere; in 113 Double-A innings, he’d struck out just 51 while walking 46. Ironically, his bat got him onto a big-league mound, in a way. As an outfielder on the Astros, he came into a game as a mop-up reliever in a blowout. He allowed two runs in an inning.

There’s a pretty good chance Bogusevic will get a few more ABs in the majors, but even if he doesn’t, he has played more big-league games as a hitter than six actual position players drafted in the first round of his class. —Sam Miller

9. Babe Ruth Becomes a Slugger
Perhaps the most infamous—and successful—position switch came at the dawn of the Roaring '20s: In 1919, the Boston Red Sox gave their hot-shot young hurler, Babe Ruth, a bat to swing on days he wasn't starting, and the 24-year-old turned in a .322/.456/.657 line with 29 homers in 543 plate appearances. The previous season, the Bambino hit a league-leading 11 homers in just 382 PAs.

But to keep Ruth as a starter or a hitter? Between 1914 and 1919, the southpaw compiled an 89-46 record over 158 games, tossing 1190 1/3 innings and collecting 3.7 strikeouts per nine for Boston, though his stats tailed off as he received more hitting opportunities. The enviable decision became the Yankees' responsibility when the Red Sox sold Ruth to New York during the offseason. The Bombers quickly moved their new slugger into the heart of their lineup, with nary a glance at putting Ruth back on the mound.

It seems that the Yankees got pretty good value out of the Sultan of Swat: Ruth went on to hit .349/.484/.711 between 1920 and 1934, famously procalimed he had a better year than sitting president Herbert Hoover, (arguably) called his shot during the World Series, single-handedly outslugged entire teams, held the career home-run record until 1974, played a huge role in reviving baseball in the '20s. That kind of success is worthy of a gold star or four. —Stephani Bee

10. Esteban Loaiza Adds a Cutter and Becomes an Ace
In September of 2002, Esteban Loaiza allowed 25 runs in 30 2/3 innings, striking out only 12 batters. He finished that season with a 4.88 career ERA, below average even in a high-offense era. He wasn’t an innings eater, either; he was just a back-of-the-rotation guy with better stuff than his results suggested. So when he turned 31 on New Year’s Eve, Loaiza was still looking for work. Finally, in late January of 2003, he signed a minor-league deal with the White Sox that paid him $500,000 when he made the team out of spring training.

And then, for one season, Loaiza pitched like an ace. He threw 226 1/3 innings with a 2.90 ERA, won 21 games, and led the American League with 207 strikeouts. He finished second in AL Cy Young voting, and second in PWARP. All of his peripherals passed the smell test. The only fluky thing about his performance was that he’d never pitched so well before.

The secret: Loiaza started throwing an effective cutter, with which he was able to change speeds. The recipe for success is almost never that simple—take fringe arm, add new pitch, mix until top-of-the-rotation starter. And it probably wasn’t quite that simple in Loaiza’s case, either: he’d recently resolved some off-the-field distractions, so he may have been more focused on the field, and contemporary analysis by ex-players indicates that he also improved his command. But Loaiza’s story is still tantalizing, since it suggests that any struggling starter could be one pitch away from a near-Cy Young season.

Loaiza couldn’t sustain his success, even to the extent that, say, Mike Scott did after he added his splitter/emery board. As quickly as he’d risen to prominence, age took its toll and sent him back into obscurity. Loaiza lost some velocity, suffered some injuries, and no longer looked like the same guy. His ERA after 2003 was 4.81, which was worse, relative to the rest of the league, than his ERA prior to 2003. But even though he didn’t last long at the top, Loaiza remains one of the purest examples of a pitcher transforming himself by adopting a new offering at an advanced age. —Ben Lindbergh

11. Willie Mays Hayes Hits for Power
During the Indians’ first playoff run under manager Lou Brown, Willie Mays Hayes was a slap hitter whose speed allowed him to beat out base hits. He kept the ball on the ground by necessity, since he was prone to popping up when he tried to elevate.

Hayes’ combination of contact and on-base skills, ability on the basepaths, and superb defense in center made him a valuable player, but he wasn’t satisfied with the status quo. Following the Indians’ sweep by the White Sox, Hayes tore down his game and built it back up again. By the time he returned to camp the next spring, he’d become a completely different player.

Hayes’ stances in his first and second springs looked more or less the same:

But the new swing was explosive, with superior weight transfer and hip rotation. Watch his back foot stay still in the first clip and swivel in the second:

Suddenly, Hayes was not only swinging for the fences, but hitting balls over them. And the power boost was perfectly timed, since he'd also lost a step, at least early in the season.

Plenty of players lose speed and add power as they age, but few do it so dramatically, or so suddenly. It was as if Ben Revere had shown up in Phillies camp this season hitting like Ryan Howard.

And it wasn’t just about the way Hayes played. It was also the way he looked and acted. He drove a new car, wore more jewelry, and turned his cap around so that the brim faced backwards. He was more difficult to coach, and less of a positive clubhouse presence. Sometimes he seemed like a different person:

The only explanation Hayes offered for his remarkable transformation was that he’d “bulked up” over the winter, which seems a little suspicious in light of what we’ve learned about baseball’s PED problems at the time. I’m not saying he used. But you have to at least ask the question. —Ben Lindbergh

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Smoky Joe Wood shold make the honorable mention.
And Reb Russell for the same reason.
Willie Mays Hayes has me cackling like a madman.
Doesn't Rick Ankiel have to be in there somewhere?
He certainly qualifies. We never include everyone who could theoretically be included, so it's not an attempt to be comprehensive.
You cannot mention Mahay and forget to throw a line about his replacement player status. While I don't think he would have ever been a star, I do remember reading some baseball notes in the 90s/00s about this fact and his lack of reception in clubhouses.
Dude, #1 on the list was Archibald "Moonlight" Graham becoming a doctor, no?
What was Alex Gordon's transformation?
How many reinventions does it take to get Bob Lemon on the list? He made it to the majors as an infielder before WWII, returned from the war to play center field for Feller's no-hitter in '46, THEN switched to the mound and had a Hall of Fame career as a pitcher. Got fired as manager of the White Sox early in '78, but replaced Billy Martin managing the Yankees a few weeks later -- and won the World Series.
Craig Biggio
Major Leaguers Dave DeBusschere and Danny Ainge became basketball stars. (Not saying they needed to be included, just thought they should be mentioned somewhere, if just here). Chuck Connors became a TV star. Did Wesley Snipes take steroids for his role? Uh, talk about dedicated actors.
If anyone did, it would've been Omar Epps.
Frank Tanana
What exactly was it that enabled Billy Taylor to transform from mediocre AAA starter in the Braves system to a closer with Oakland in the mid 90s? Also, not a positional transition, but IIRC Bill James recounted how Wally Moses reinvented his value a few times over his career - for example, stealing 56 bases as a 32-year old in 1943, after never having stolen more than 16 in a season.
Frank Tanana went from a hard drinking hard throwing strikeout machine to a devout christian relying on guile and finesse. Tommy John let some doctor talk him into a crazy surgery where they took a tendon out of his leg and grafted it onto his elbow. It added over a decade to his career thought to be over and got the surgery named after him. Surely one of these guys would have made the list fictional characters were excluded. ;-) And if we're gonna include fictional characters, how can you ignore Ricky "Wild Thing" Vaughn getting glasses? ;-)
Perdo Cerrano's conversion from Voodoo to Buddhisim worked out in the end. Struggled at first though.
Not the kind of conversion I was thinking about, but yes, good point.