It is a shame. But I don't think he'd've been right for the part. If Michael Gambon hadn't taken over the part of Dumbledore then he would've been alright in it.
Well, I saw him while watching the Indiana Jones dvds over the weekend, and thought he'd make a good Mad-Eye. But maybe he'll be a bit high profile now after Lord of the Rings.
I just checked the imdb and she's not amongst the characters in Azkaban, so that's part of the book they've cut out for the film. I hope they don't ruin it too much, Azkaban is my favourite.
Anyway, as to who to play Cho in subsequent films, I'm not sure. I can't think of many young British-Chinese off hand, I'd have to give it some thought.
Who cares she is just going to be a brand new child star so why bother trying to cast it.
As for the cho chang thing it was announced ages ago when they started filming she wasn't going to be in it. IMDB doesn't tell you much you know it's not the be all and end all of sources.