Harry Potter films, some quieries.?

My guess is that, as the directors and other crew change, the overall feel and style of the films change as well. It's not just Dumbledore by the way, I'm sure you've noticed that the films themselves are getting darker and grungier. The books got darker too, but it's harder to convey the same depth of change in a two hour film, so they have to go with strong visual differences. As to Flitwick looking younger, I haven't really noticed. The actor's the same, so chalk it up to more directorial differences.