Owen had originated the role of the normal Munster, Marilyn, for the first 13 episodes. And she was miserable. She was very depressed and often seen crying on set. The story is, she was trapped by her contract to the studio and never wanted to be a part of the now-classic show. It meant that she had to be away from New York, and from her soon-to-be husband.
Owen was released from the contract, moved home, got married and had two kids. So, Pat Priest seamlessly took over the role. It helped that Priest looked so similar to Owen that plenty of viewers didn't even notice the change.