The Goldbergs
The Goldbergs was a beloved show, so when Alexis G. Zall replaced Rowan Blanchard in the sixth season, fans were understandably upset. However, the truth was that Rowan was sick of the part and wanted out of her contract. Youtuber Alexis G. Zall was hoping to shed her online image and move to TV. It worked out great for everyone but the viewers...
Pretty Little Liars
Deception is the whole theme of this show, so it's no surprise that Toby Cavenaugh was actually played by James Neate during the first few episodes. He even had a different name--Toby Marshall. After Keegan Allen took over the part, the name change matched a recasting. Neate left the popular drama to star in the Amazon show The Man in the High Castle, which received high acclaim.