Tom Ellis As Robin Hood In "Once Upon A Time", Replaced By Sean Maguire
The production was obliged to replace Tom since, at the time, he couldn't commit to portraying Robin Hood and had other obligations to complete. Sean Maguire got the role of Robin Hood, and as far as the public knows, there is no cold blood between Tom Ellis and the production executives.
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.