Matthew Lewis
While all of the main actors of Harry Potter changed drastically over the years (they were mere children when the first HP movie came out), no one displayed such a dramatic change as Matthew Lewis. No offense to Matthew, but he wasn’t the best-looking guy in HP, and that’s saying quite a lot.
However, that came to an end when a. Cedric Diggory died and b. he got dental reconstruction surgery while filming the first half of Deathly Hallows. He allegedly breached his contract when he got his teeth fixed, but that didn’t matter—he was long past his days as Neville Longbottom at that point.