Oh, damn. What kind of changes? I noticed the preview looked a bit...off, but I never put much stock in previews (plus I was waiting for HP5 to start, so I wasn't paying much attention).
The family's American, instead of British.
The parents are career-oriented to the point of almost ignoring their kids.
Will's 13 (remember the book began on his 11th birthday)
Will fights with his brothers.
Max (the older brother in art school, who was a minor character in the book) is Will's best friend and role model.
Speaking of Max, he's pierced and tattooed and very defiant of the father.
Merriman, who was described as tall, gaunt and sharp-nosed, is being played by Ian MacShane. (I love the actor, but he is not Merriman.)
The Arthurian themes are all but wiped out.
Here's a better breakdown of most of the changes:
http://community.livejournal.com/authorblog/5880.html
It breaks my heart, since I've loved the books for over twenty years.