Actually, I don't know if it works that way. SM has said that she was practically a producer on Eclipse, and that didn't turn out well, in my opinion. It's clear that she lost some battles in that one, although hopefully, considering the backlash to the motorcycle scene, they won't do anything as contentious as that again, especially now that the love triangle is effectively over.
I don't mind some changes. It's difficult to translate a book, especially ones as wordy as the Twilight books, to a visual medium. Some of the changes can be quite enjoyable. So long as they don't make the characters do things that aren't in character.
I'm actually hoping to see a bit of Tanya crushing on Edward, perhaps a flashback to when they first met. I'd love to see him resisting her advances! Especially if it gives me what I really want: jealous Bella!