With all the talk for JDS' need to shake up his style, has there ever been a fighter that really evolved in a big way mid-career?
I'm talking like guys that took something that wasn't their natural skillset and really improved it mid-career -- say, at least four fights into a UFC career. I don't mean guys like JBJ that showed aptitutde for everything, I mean guys that couldn't strike or wrestle for sh*t that really turned it around mid-career.
Bisping's TD defence is vastly improved compared to what it was pre Hamill. 
Might be later than mid, but MIKE PYLE.
Maldini2706 -Bisping's TD defence is vastly improved compared to what it was pre Hamill.
Agree with you there, and his offensive wrestling got much better. Still, Chain couldn't give him a centimeter and he was up.
His newer style brought detractors but GSP significantly changed his game mid-career
I think Werdums striking has come on light years in his past few fights
R Evans FO SHO!
He's come a long way
Nog's punching power has dramatically increased.
Maia adopted that strike first style for a few fights but has gone back to bjj
Randy Couture vastly improved all areas of his game over his career.
Matt Brown was a .500 TUF reject who is now on a 4 fight win streak, in possibly the most talent rich division in MMA.
Matt Hamill got way better at striking than when he started
Cub Swanson's always been good but he's turned it up considerably as of late.
what would sakuraba do - Anderson silva. Forever he was a very thin, weak physically but technical striker. Bjj guys would throw him around for years until 05-06 he gained 30lbs muscle, got much stronger, started one shot koing muhfukas left and right. He used to have a normal sized head as well. Latent Cranial Gigantism?
Kenny Florian
Chris Lytle
Matt Serra
Chael
Nate diaz
Nate Diaz going from mainly a grappler to a well developed striker.
Matt Serra
Overeem "evolved" into an entirely different species.
Seriously though, I'd say Anderson Silva is the best answer. He suddenly went from fringe contender to GOAT like 10 years into his career when he was already over 30 years old.