Eric Stoltz As Marty McFly In "Back To The Future", Replaced By Michael J. Fox
Marty McFly wouldn't be the iconic character without the involvement of Michael J. Fox. Because of scheduling conflicts, Eric Stoltz got cast as Marty. However, his portrayal of the character and overall bad comedic timing was not quite what director Robert Zemeckis was looking for, and he effectively got recast.
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.