Wouldn't be surprised if Dumbledore was CGI - they've got enough footage of Richard Harris to do so, CGI is now of a quality where this is possible, and he hardly shows up in the films anyway - it'd save using a new actor which is oh-so-annoying once you associate a face with a character.
Well, the movie is out now and the casting was wonderful. You were right about Gary Oldman, he is Sirius, but the others were wrong. You probably know all about this by now tho.