First name that came to mind for me was Stan Mikita. Scrappy type of player who put up a lot of points and turns into a back to back Lady Byng winner. Check this out, here are his penalty minutes around his transformation:
1964: 146
1965: 154
1966: 58
1967: 12
1968: 14
1969: 52
He won the Art Ross and Hart in 1967 and 1968 as well. Had another year of 85 PIM after this, but was generally around 40-45 PIM a season the rest of his career. I have no idea what happened after 1966. Did he just feel like he was better off being on the ice as opposed to the penalty box and he cleaned up? It sure helped the Hawks in the standings, they had 94 points in 1967, far and away the best in the NHL. I am just not sure why those two seasons stick out. He didn't rack up the PIM after 1968 or anything, but he also wasn't close to having that few of PIM in a season. Either way, a heck of a transformation.
Scott Stevens was already rightly mentioned but how about Red Kelly? Guy literally changed positions despite already having a lock cinch Hall of Fame career as a defenseman. Goes onto win 4 Cups as a forward and cements himself as the player with the most Cups in NHL history of a player who never played for the Habs. And look at those playoff numbers, 1962, 1963, 1964 especially. Red was a machine on that team.
How about Yzerman? Would post-1995 or 1996 come into play here? Goes from the wild offensive numbers to being still good offensively but willingly sacrificing offense for better team play and defense. But let me also throw this in here that Yzerman was never one dimensional. Killed penalties throughout his whole career, led the NHL in shorthanded goals 3 times, etc. Blocked shots and was probably a rare star that did it. And this is all before his mid 1990s change. He just sacrificed his offense and played a more well rounded game, but I never thought he was bad defensively.
Eric Lindros after the Stevens hit was different. I'll admit though he could still fight. We saw this in 2004 when he belted Thornton and probably ironically propelled him into his own transformation. But in a bad way Lindros became a shadow of himself after 2000. This was a Lindros that played the perimeter as a New York Ranger. Not the overpowering guy that threw his body around and struck fear into everyone even without the puck. He could still fight as we know, but he very much played like a guy on the ice who didn't want to get hurt again.