Game of Thrones
Game of Thrones seemed to just swap out actors whenever they felt like it, huh? The mythical Three-Eyed Raven is no exception, first being played by Struan Rodger, with Max von Sydow taking over soon after. The first iteration of the birdman didn't do much, so no one really noticed until they checked IMDb.
The Fosters
Talk about replacing one big star with another! When Noah Centineo was still just another struggling actor/ heartthrob, he got a job replacing Disney Channel's Jake T. Austin on The Fosters. Jake was pretty upset at the sudden change, commenting on his new absence by saying, "I was only asked to return for three episodes and wasn’t going to be able to work on anything else, so I left..."