OSC comments on some changes from the novel (Potential Spoilers)
On Hatrack, OSC briefly mentioned that he's considering writing a new version of EG, and a change he's had to make for the sake of it being a movie:
Ender's Game, while at the moment it is still my most popular, is not my best novel. I've learned a few things since writing it. Indeed, after working on the screenplay, I'm soooooo tempted to create a completely new version of Ender's Game that includes some of the really cool stuff that I've come up with for the film script. (While still not be a movie novelization - for instance, I still like having Ender meet the hive queen on another planet, not on the asteroid Eros, but for the film there's just no way.)