The Partridge Family
Brian Forster was the one playing Chris, but he was replaced later by Jeremy Gelbwaks after their family moved. But that’s far from the whole story. It doesn't end there though. Allegedly, Jeremy was said to have “had a personality conflict with every person in the cast and the producers”—which is a hard stance to take against someone who can't even multiply by 12 yet.
Dallas
Jenna Wade from Dallas was not just any character, so the role couldn't go to just any actress. Morgan Fairchild acted as Jenna for a single episode, but she was replaced by Francine Tacker for two episodes. Then, when Jenna returned a few seasons later, who took the part but the wife of rock n' roll, Priscilla Presley?