Claudia Wells As Jennifer In "Back To The Future", Replaced By Elisabeth Shue
In the original movie, Jennifer was seen only at the start and at the end, and Claudia Wells got the role. When it came time to film the sequel, Wells did not reprise her acting role due to family problems. Forced to recast the role, Elisabeth Shue auditioned and got the part of Jennifer.
Also, how would anyone be able to tell when some background character in GoT was replaced? There were a ton of them, and they all looked the same.