"The Immortal" Hulk Hogan to "Hollywood" Hulk Hogan. No one could imagine Hogan as a heel again after the Hulkamania run of the 80's. People thought Hogan bringing his stale act to a new promotion like WCW and having new evil doers to run through would make him somewhat fresh again, but the initial buzz wore off pretty quickly and it was looking like his wrestling days may have been done for good, and that's what some people seemed to want. But then he made the magical transition from the read and yellow superhero to the guy who was leading a gang of renegades from WWF (the same place he put on the map in a way) and he started calling himself "Hollywood" as a way of him soaking up just how big of a legend and icon he had become.
The Ringmaster to "Stone Cold" Steve Austin. Yeah, he technically debuted the Stone Cold character in ECW (he even says "that's the bottom line" in at least one promo), just without the official name, but still, they made him the Ringmaster when he came to WWF and he would have gone nowhere with that. Stone Cold allowed him to become one of the greatest ever. Even if you look at his work in WCW as "Stunning" Steve Austin, you could have never imagined him as someone who would end up being the epitome of a bad ass in WWE and wrestling in general. Vince even says as much on his DVD when he points out that he could see he had charisma in WCW but not to the extent of imagining him as a top hand. He's the one who gave him the Ringmaster moniker, after all.
Bradshaw to JBL is a neat one. I loved APA Bradshaw but I never envisioned Brashaw as a main eventer and the JBL character was a perfect fit to make him one. I loved his 2004/2005 title reign, unlike most at the time.
Issac Yankem to Fake Diesel to Kane. You go from being a cartoonish evil dentist to being the very bad caricature of someone else to being the physically/mentally scarred brother of the Undertaker, one of the greatest ever? The introduction of Kane and the mystique they built up for him leading to his debut and the debut itself and the story of Kane and Undertaker in 1998 is still one of my favorite ever.
"Mean" Mark Calloway to the Undertaker. Is there really an explanation needed for this one? You go from a average tough guy character (though that heart punch was awesome) to being the most unique character/gimmick ever.
Rocky Maivia to The Rock. He had charisma as Maivia, sure, but who could have imagined that this guy would be the most charismatic ever who could hold any crowd in the palm of his hand and be the absolute epitome of an arrogant ass hole?