Once Upon A Time
The storybook show had quite a few changes in characters over the years. Tom Ellis originally played the fictional Robin Hood in season one, but Sean Maguire stepped into the role for the remaining seasons. Ellis just had a few scheduling conflicts but harbors no ill-will toward Maguire.
Rosanne
Read closely for this one: Lecy Goranson left Rosanne to go to college full time. She played the role for five years before being replaced by actress Sarah Chalke from the show Scrubs. Lecy then left college and came back to the program, only to be replaced by Sarah again. They did a short-lived reboot which Lecy starred in. Phew!