He's not even the first mover this offseason -- Carlos Beltran, for instance, left the Rangers for the Astros.
It remains to be seen how these free agents will fare with their new teams -- and against their old ones -- but MLB.com can look back at some of the biggest names to switch sides of a rivalry since the introduction of free agency in 1976.
Right-hander Luis Tiant set the precedent in 1978, when he jumped from the Red Sox to the Yankees following a frantic race for the American League East title, which ended in a one-game tiebreaker, punctuated by Bucky Dent's dramatic go-ahead home run for New York.
Rival-to-rival moves haven't been limited to players. Hall of Fame manager Whitey Herzog left the Royals for the Cardinals after the 1979 season, managing St. Louis as the in-state rivalry blossomed during the mid-1980s. The Royals got the better of Herzog and the Cardinals in the 1985 World Series, while Herzog won a championship of his own with the Cards in 1982.
Here are some of the others who have switched sides since:
Jason Heyward and John Lackey (Cardinals to Cubs, 2015)
Outfielder Heyward signed an eight-year, $184 million contract with the Cubs following the 2015 season, leaving the National League Central-champion Cardinals for the rising Cubs, who had beaten St. Louis in the NL Division Series. Heyward struggled badly at the plate, but his defense helped the 2016 Cubs win their first World Series since 1908. Right-hander Lackey was a less pricey, but important acquisition for Chicago, becoming a workhorse in the rotation.
Daniel Murphy (Mets to Nationals, 2015)
Even after Murphy's historic postseason run in 2015, the Mets let the slugging infielder walk in free agency. He signed with the Nationals, who had lost to New York in the NL East race, and became one of the biggest reasons for the clubs' reversal in the standings this season. Murphy finished as the NL MVP runner-up and tormented his former team -- batting .413 against the Mets with seven home runs and 21 RBIs in 19 games, hitting safely in every one.
Jacoby Ellsbury (Red Sox to Yankees, 2013)
Like Heyward, Ellsbury was an outfielder who received a huge payday from a rival, as the Yankees lured him from Boston to the Bronx with a seven-year, $153 million deal after the 2013 season. Ellsbury's production has also dropped off -- in his three seasons in New York, he has a below-league-average 95 wRC+, compared to his 109 wRC+ over seven seasons with the Red Sox.
Josh Hamilton and C.J. Wilson (Rangers to Angels, 2012 and 2011); Vladimir Guerrero (Angels to Rangers, 2009)
The emergence of this AL West rivalry largely coincided with the movement of star players between the teams. After the Angels won three straight AL West titles from 2007-09, Guerrero signed with Texas and became an All-Star outfielder in 2010 for the first time in three years, as the Rangers took back the division title and reached the World Series. In the years following, left-hander Wilson and outfielder Hamilton left playoff teams in Texas for Anaheim. The Angels got back to the postseason in 2014, but the pair did not live up to their contracts. Hamilton had a particularly messy falling out and ultimately returned to the Rangers.
Jason Schmidt (Giants to Dodgers, 2006)
Schmidt was coming off three All-Star nods in four years, but the Giants hadn't made the playoffs since 2003 when the right-hander signed with the Wild Card-winning Dodgers after the '06 season. After he signed with Los Angeles, everything went south -- Schmidt soon had to undergo major shoulder surgery, lost his velocity and managed only 10 starts before retiring after the 2009 season.
"It's all a little disappointing," he said of his time with the Dodgers. "It was nice to get back and pitch [a few] more games. There was a little closure that way."
Johnny Damon (Red Sox to Yankees, 2005)
A Boston fan favorite who helped the Red Sox shatter the Curse of the Bambino in 2004, outfielder Damon shocked Red Sox Nation when he signed with the Yankees following the 2005 season. The Red Sox did not match New York's four-year, $52 million offer, with Damon saying George Steinbrenner and Brian Cashman pursued him aggressively.
"I made contact with [the Red Sox] and told [Terry Francona] they really need to get going, because if not, I'm going to be on another team," Damon said. "I'm not sure if they knew I meant it, but now I'm a Yankee."
Damon helped the Yankees win their 27th championship in 2009, while Boston has won two titles since he left.
Magglio Ordonez (White Sox to Tigers, 2004)
Outfielder Ordonez leaving the White Sox for their AL Central adversary led to a feud erupting with fellow Venezuelan native and hot-tempered Sox manager Ozzie Guillen. They traded verbal jabs, with Ordonez eventually calling Guillen "my enemy," and Guillen responding with a profanity-laced tirade. Ultimately, the move didn't quite pay off for Ordonez, as the White Sox won the World Series the year after he left and Ordonez lost his only Fall Classic with Detroit in 2006.
Tom Glavine (Braves to Mets, 2002)
Even with the longstanding Mets-Braves rivalry, Atlanta franchise icon Tom Glavine headed to New York in 2002, when the Braves didn't match competing offers for the two-time Cy Young winner and 1995 World Series MVP.
"It's almost surreal that it happened," Glavine said. "I never thought I would play for someone else. I thought I would play my whole career with the Braves."
Glavine went on to make two All-Star teams with the Mets and helped them come within a game of the World Series in 2006.
Kevin Brown (Padres to Dodgers, 1998)
Right-hander Brown spent only a year in San Diego before jumping ship for the big-market Dodgers -- signing a then-record seven-year, $105 million free-agent deal in 1998. That did not sit well with Padres fans, who booed Brown mercilessly in his return to Qualcomm Stadium. Owner John Moores, who had offered Brown a six-year, $60 million contract, had the stadium play the Beatles' "Can't Buy Me Love" as Brown warmed up, and said before the game, "I hope we clean his clock."
San Diego did, in fact, beat Brown in the game, although Brown went on to be an All-Star twice in Los Angeles and led the league in ERA in 2000.
Albert Belle (Indians to White Sox, 1996)
Outfielder Belle led the Indians to the World Series in 1995 and a division title in '96, but the divisional-opponent White Sox nailed down the marquee free agent on the market that offseason by making the slugger the highest-paid player in baseball. Belle mashed 79 homers in his two seasons in Chicago, earning an All-Star nod and a Silver Slugger Award, but the White Sox did not make the playoffs either year.
Wade Boggs (Red Sox to Yankees, 1992)
The Hall of Fame third baseman was on a run of eight straight All-Star seasons when he became a free agent in 1992, but his batting average had fallen below .300 for the first time in his 11-year career, to .259. The Yankees, banking on the five-time batting champ regaining his form, inked him to a three-year, $11 million deal. Boggs did bounce back, hitting above .300 the next four seasons, making four All-Star teams and winning two Silver Slugger Awards. In 1996, he helped ignite the Yankees' dynasty, winning the only World Series title of his career.
Nolan Ryan (Astros to Rangers, 1988)
The Hall of Fame strikeout king and Texas native had been pitching for the Astros for almost a decade when he became a free agent as a 41-year-old in 1988. A contract dispute with Astros owner John McMullen caused Ryan to leave for the in-state Rangers, with whom he led the league in strikeouts twice, including 301 at age 42. Since retiring, Ryan has been involved in the front office for both organizations -- he served as Rangers president from 2008-13, and he now works for the Astros as a special assistant to owner Jim Crane.
David Adler is a reporter for MLB.com based in New York. Follow him on Twitter at @_dadler. This story was not subject to the approval of Major League Baseball or its clubs.