2. David Michael Scott (Beric Dondarrion)
The Replacement: Richard Dormer
David Michael Scott was initially hired to play Beric Dondarrion well before he was of any real interest to the show's major plotlines. He was probably best considered a stand-in character, only briefly seen on screen, before the show really needed to cast someone more notable. Indeed, Scott was only credited as a Featured Extra, initially.
When Beric became a more key figure in Arya's story, Scott was replaced by the excellent Richard Dormer, transforming a supporting non-entity into one of the low-key best characters in the entire show.