recast

When a character in a movie or TV series is played by someone, but is later replaced by another actor/actress in the story's continuity.

Bob: You ever seen The Incredible Hulk?
Joe: Nope. It looks lame.
Bob: It takes place before The Avengers.
Joe: How? The actors who play Hulk in are different.
Bob: It was because of a recast. Edward Norton was supposed to continue playing Hulk, but left after creative differences. He was replaced by Mark Ruffalo.