In every movie franchise, actors tend to get replaced for some or the other reason. Sometimes the actor is too expensive or maybe not good enough for the character and that's what ends up getting them replaced.
Even in our beloved movie series Harry Potter, there were 12 actors who got replaced and possibly most of the audience didn't realize that. Let's take a look -
Played by Verne Troyer in the Philosopher's stone, a goblin who shows Harry to his vault for the first time ends up being replaced by Warwick Davis in the Deathly Hallows.
Jennifer Smith, an African American actress played an extra being Lavender Brown in the third movie while Jessie Cave was given the character of Lavender Brown by the 6th movie. There was a big controversy regarding an African American actress being replaced by a white actress but that's all in the past now.
British TV veteran Peter Cartwright played Doge in the Order Of The Phoenix as one of the guards who was supposed to move Harry to The Grimmauld Place. Later in the DH, David Ryall had replaced Cartwright.
It was another one of the background actors being replaced by an actual actor when there is more performance to come. Emily Dale was replaced by Georgina Leonidas by the 6th movie.
Derek Deadman played the character in the first movie but was replaced by Jim Tavare since he expired later on.
Played by Elizabeth Spriggs in the Philosopher's Stone, she was replaced by comedian Dawn French to add a little comic twist to the character by the third movie.
Played by Scott Fern during the first two movies, the actor was later replaced by Jamie Yeates. Owing to his innocent sweet looks, Fern was replaced and its so noted that Yeates does more justice to the role.
Nina Young was replaced by Scottish Actress Kelly MacDonald after Helena was supposedly to tell Harry about the diadem horcrux.
Played by Genevieve Gaunt as an extra in the first few HP movies and later played by 4 different actresses. Finally Scarlett Byrne played Pansy, as a pug faced female who was another bully from Slytherin.
Jessica Hynes' voice over was there in the initial film where she was supposed to read a howler out to Harry who had performed underage magic but Sophie Thompson went on later to play the character for the Deathly Hallows.
Richard Harris played Dumby in the first two movies and later on the role was taken on by Michael Gambon. It was because Harris passed away before the premiere of Chamber of Secrets that lead to the recasting.
In the Philosopher's Stone, when Voldy shared body with Quirrell, the character was played by Richard Bremmer who was later replaced by Ralph Fiennes to play the antagonist in the HP movie franchise.