When the Harry Potter movies started releasing, nobody could have guessed the phenomenon that it would become. The movies had a star-studded cast from the beginning, but many seemingly minor characters were recast over the course of eight films.
Since all the books were not released when the movies were, there was no way of knowing when a minor character would become a major player. Thus, more experienced actors were brought in for characters that became important later, and many characters got recast for sundry reasons like scheduling conflicts and availability.
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10 Albus Dumbledore
Richard Harris' Dumbledore was supposed to prepare Harry for his final battle against Voldemort, but unfortunately, the veteran actor passed in 2002. He portrayed the powerful wizard in Harry Potter and the Sorcerer's Stone and Harry Potter and the Chamber of Secrets, after which Michael Gambon took the mantle.
Gambon added his unique flair to Dumbledore from Harry Potter and the Prisoner of Azkaban until the very end of the franchise. Both played the part excellently.
9 Vincent Crabbe
Crabbe may not have been the smartest or strongest Slytherin in Harry Potter, but he was important to the story, especially in Harry Potter and the Deathly Hallows Part II. Jamie Waylett played Vincent Crabbe in the first six movies, but after he ran into some legal trouble, Warner Bros. removed him from the franchise.
Instead of recasting Crabbe in the last movie for the pivotal scene, the makers replaced his character with Blaise Zabini, portrayed by Louis Cordice. The actual Fiendfyre was then lit by Goyle (Josh Herdman) in the movies instead of Crabbe.
8 Pansy Parkinson
Draco Malfoy's sidekick didn't have much of a meaty role in the movies but was always lurking with the Slytherins while they bullied Ron or Harry. Thus, Parkinson was recast a whopping four times across the eight Harry Potter movies.
In the first two movies, Katherine Nicholson was the brash and irritating Pansy, and Genevieve Gaunt took over in Harry Potter and the Prisoner of Azkaban. Lauren Shotton took on the part in Goblet of Fire. Shotton and Nicholson were uncredited. Finally, Scarlett Hefner was cast as Pansy in the fifth movie, and she remained till the end.
7 Lavender Brown
The recasting of Lavender Brown was a controversial decision taken by Warner Bros. The first two actresses who portrayed the part were young black actresses, namely Kathleen Cauley (Chamber of Secrets) and then Jennifer Smith (Prisoner of Azkaban.) They were likely replaced due to the age of the character.
However, when Lavender assumed a much larger role as Ron's girlfriend in Harry Potter and the Half-Blood Prince, Smith was recast, and Jessie Cave took over. The problem was that Cave was white, and fans were alarmed that the moment a character became important, they were whitewashed.
6 Lord Voldemort
Many actors portrayed Voldemort to show his arc from childhood, but adult Voldemort, the most important of them all, was recast thrice. Richard Bremmer acted in the scene in Sorcerer's Stone where Voldemort killed James and Lily, but his face was not visible in the scene.
In the same movie, Ian Hart portrayed the villain as the face behind Professor Quirrell's head, but he did not carry on in the ensuing movies. Voldemort returned to his physical form in Goblet of Fire, but the role was reprised by Ralph Fiennes, most likely because he was a big superstar.
5 Griphook
Audiences got their first look at Griphook, the wily goblin in Sorcerer's Stone, played by Verne Troyer, a rare American within the all-British star cast of Harry Potter. This was why the voice acting was done by Warwick Davis, who was Professor Flitwick in the movies too.
However, Davis was given the role wholly in Deathly Hallows Part II and II, where he played both parts with elan. He transformed when he played shrewd Griphook, a far cry from the genial Flitwick.
4 Helena Ravenclaw
At first, the ghost of Helena Ravenclaw was merely a background entity in the first two Harry Potter movies. Nina Young was the original Grey Lady, but when Helena's role became vital to Harry winning the war in Deathly Hallows Part II, she was recast, and Kelly Macdonald stepped in.
The suddenly meatier role likely required a more accomplished actor, and Macdonald is a BAFTA-winning artist. Her Grey Lady, one of the strongest Ravenclaws, was perfect for the last movie.
3 Padma Patil
In Prisoner of Azkaban, Padma is a background character played by actress Sharon Sandhu. The Patil sister was a Ravenclaw in the books but was reassigned to Gryffindor in the movies.
Padma's character became important in Goblet of Fire, where she was Ron's date to the Yule Ball after he messed up his chances with Hermione. Thus, she was recast, and Afshan Azad took over from Sandhu. Azad acted great, but her costuming could have been better.
2 Parvati Patil
Padma's sister, Parvati, had the misfortune of becoming Harry's date to the Yule Ball, the idea of which was only exciting for the first five minutes. In Goblet of Fire, Padma was portrayed by Shefali Chowdhury, but the character was played by someone else in the previous movie.
Sitara Shah's Parvati had a memorable scene in the third movie, where she fought off a Boggart in Defence Against the Dark Arts. She was much younger looking than Chowdhury, which could be a reason for the recast.
1 Bill Weasley
Bill Weasley was not a big part of the movies for a long time. In Prisoner of Azkaban, he only appeared in the Weasleys' Egypt trip photo and was played by actor Richard Fish. In Deathly Hallows Part I and II, Bill had a much meatier role, and Fisher was recast.
Domnhall Gleeson, son of Brendan Gleeson (Mad-Eye Moody), got lucky when he stood in for one of the Weasleys when he visited his father on set. He portrayed Bill as he got attacked by Greyback, got married, and fought as a senior member of the Order of the Phoenix.