The Waltons
You probably don't remember the television show The Waltons, so you almost definitely didn't know about their casting change. Richard Thomas played the character of John-Boy, a kid who graduates from college and moves to the Big Apple to become an author. As he aged into the role, Robert Wightman replaced Richard Thomas.
Batwoman
Will the city notice if their hero suddenly looks different overnight? Actress Ruby Rose played the part for only one season before leaving the show. She was then promptly replaced by...no one. This show hasn't been cast yet, but producers definitely want to do a season two. Once one signs on to the series, it'll pick up where it left off.