Back to the Future II
George McFly is pretty absent throughout the second movie, and that was for good reason. Crispin Glover left the franchise due to a dispute over his paycheck. He was replaced with Jeffrey Weissman, who was only featured a few times because of how much time and makeup it took to make him look like the original George. Details of this contract dispute changed the laws of actor rights.
Harry Potter
It would be pretty hard to catch this change without looking up the credits. After all, without a nose or any real facial features, this monster looks barely human at all! Voldemort is the scariest villain in young adult literature, originally played by Richard Bremmer, he was fired for the replacement of Ralph Fiennes.