ABC Archive/Getty Images; Carsey-Werner/courtesy Everett Collection
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Sarah Chalke and Lecy Goranson, Roseanne
You won't find a more complicated switcheroo than this one. Goranson left the series at the beginning of the fifth season to attend Vassar, but made sporadic appearances as Becky. Producers eventually decided to recast her with Chalke, a move that didn't go over well with fans because she lacked Goranson's blue-collar, Midwestern appeal. Even worse? Goranson later came back, and she and Chalke alternated the role before Chalke took it over completely in the final ninth season. The show often made inside jokes about the swap. "I can have you replaced, you know," Roseanne told Becky No. 2 once.