MS
1%er
Marty Reasoner jumps to mind. Was supposed to be a soft scoring forward and turned himself into a gritty shot-blocker.
Yeah, this is the one that always stands out to me. Went from being considered an absolute creampuff to one of the grittiest defensive centers in the league.
Kyle Wellwood's transformation wasn't quite as polar but he's re-invented himself quite nicely as a reliable 3rd line center who does a great job defensively.
Jamie Rivers was one of the most high-risk/high-reward defenders you'll ever see when he was in junior - scored 121 points one year, was nicknamed the 'Riverboat Gambler'. To stick in the NHL, he turned into the most boring, low-risk 3rd pairing defender you can imagine, and averged about 9 points/82 games over his career. That was always one of the weirdest to me - was like his talent just disappeared.
Bouwmeester was considered a guaranteed #1 dman who can do it all. He was said to be as smooth of a skater as Scott Niedermayer, with the ability to control a game with his play. Today, he is far from that. He's at best a decent 2-3, defensive dman.
As someone who said that Bouwmeester projected as a 'taller Bret Hedican' when I was watching him in junior, he turned out absolutely exactly as expected.
Having said that, he's pretty clearly a 1-2 defender who plays 27 minutes/game and is very good defensively and on the PK ... just absolutely sucks on the PP. Unfairly maligned player who is the least of Calgary's problems.