Dick York as Darrin suffered a back injury be previously suffered while filming the movie They Came to Cordura became too much to handle. He handed the role off to Dick Sargent and the show continued. Since the two resembled one another, no one really complained.
This replacement was a huge plan in the script itself. The character of Ann was mocked for being so forgettable by other characters on the show. In fact, she was so forgettable that Alessandra Torresani originally played her and was replaced by Mae Whitman. This was all part of the joke.
Last Man Standing
Between seasons 1 and 2, a new showrunner was brought in to improve ratings. Apparently this showrunner and the cast had some creative differences, so they were simply replaced and the show went on.
Shailene Woodley played the role of Kaitlin Cooper, younger sister to Marissa, played by Misha Barton. But after the pilot, she was replaced by Willa Holland.
The iconic role of Miss Ellie on Dallas was originally played by Barbara Bel Geddes, but in 1984 she took a break for bypass heart surgery. She decided to focus on her recovery and Donna Reed was brought in to replace her. But viewers weren’t happy with the change, so Barbara was eventually brought back.
Because the artists are giving their voice its easy to miss this change. Meg was originally voiced by Lacey Chabert, who had to leave because of school and her time commitments with Party of Five. She replaced by Mila Kunis and the show went on.