First of all, Columbus did not do a good job with the series. The things that were good about the two Harry Potter movies were good in spite of him, not because of him. He only ever planned to do the first two movies in the series because it requires working in England, and he was unwilling to relocate his family there semipermanently.
I don't think there's an explicit intention to switch directors on each film from here on out, but they may feel that this will happen because a) few directors want to become chained to a series, no matter how profitable it is and b) few good directors want to do all there work in and around London. My guess is that Cuaron will do a good job with Azkaban (which will do well because interest will be renewed next year with the release of Book Five) and will be rehired (if he's interested) for Goblet of Fire. There will definitely be a third director in by the fifth movie, though. That director? Peter Jackson, if there's any justice at all in the world.
As for Dumbledore, I had another good idea (not that any of the Dr Who's wouldn't do a decent job). I think Jim Broadbent would be good - not as he appeared in Moulin Rouge, obviously - more like the character he played in Iris. Or what about Sir Richard Attenborough? He'd be pretty good and, now that James Coburn is dead, he's one of the only principle actors in The Great Escape that's still alive (Charles Bronson and James Garner will be dead before the end of the year at this rate).
