Adventures of Superman
Lois Lane was first played by Phyllis Coates in season 1. Taking place in the 1950s era, actress Noel Neill eventually took her place onset of the show. Though both received rave reviews, sometimes, the role just isn't the right fit at the right time. Coates just had more important commitments going on back then.
The Munsters
Have you ever heard the phrase "She's a real Marilyn Munster" before? That phrase comes from this show, where Marilyn is the only normal one in a family or oddities. Beverley Owen was the original odd-one-out, but she was replaced by Pat Priest when making the move to New York to be with her husband. Fun fact: her husband was a writer for Sesame Street!