Game of Thrones
In this show, just because your character doesn't die, does not mean your safe from losing your job. The Night King is a character prevalent throughout the series, but Richard Brake played the original Night King for the first five seasons. He was later recast with Vladimir Furdik. Vladimir also portrayed the very first White Walker Jon Snow had to destroy.
Last Man Standing
If anyone bothered to watch this side-eyed sitcom, you might have noticed Nick Jonas playing the deadbeat boyfriend in the first season. However, due to Nick's unsurprisingly busy tour schedule, he wasn't able to keep up with the demand of the job. He was replaced by Jordan Masterson and it was never acknowledged onscreen again.