The Professor Flitwick change in the third Harry Potter movie made many wonder if the Charms teacher was recast for The Prisoner of Azkaban, as his appearance is incredibly different. Instead of the old, grey-bearded professor from the first two movies, the version of Professor Flitwick audiences see in Prisoner of Azkaban is younger and sporting a mustache. In the books, Flitwick is described as a small, old wizard with a white beard, who needed to stand on a pile of books to teach.

https://screenrant.com/harry-potter-prisoner-azkaban-flitwick-change-reason/