Many movies use music in the trailers that is never used in the movie itself. This happens for various reasons. For example, the composer might not be done with his work yet, or s/he doesn't want to spoil people about what kind of music there will be in the movie.

Nobuo Uematsu has used One-Winged Angel from FFVII Reunion and Final Fantasy VII Main Theme from FFVII Piano Collections so many times in the trailers that while it is possible for those themes to appear again during the Advent Children, that doesn't mean they'll be played there for sure.

Since Uematsu has left Square Enix for now, I think he's already finished composing FFVII AC's soundtrack. According to an interview, Tetsuya Nomura is inserting the music into the movie. If there will be new scenes that need new music to be composed there, Square Enix will probably ask some other composer to help Uematsu there. It could be Hitoshi Sakimoto (who's doing the music for FFXII) or someone else, though I'd hope for Yoko Shimomura myself. She can arrange Nobuo Uematsu's songs very well, and her arrangement of One-Winged Angel in Kingdom Hearts is superb.

I think I heard some audio clips from some screening festival event, but I can't remember much of it. IIRC, the music is quite different from FFVII. The melodies feel somewhat familiar, but I still can't really recognize them. Possible AC SpoilerThe flashback scene between Cloud and Aeris had a soft theme that reminded me of Aeris' theme, but it still didn't have the same melody.

Rock and folk music, eh? It will be interesting to see how well that works out. I still hope Uematsu will put some epic, dramatic, emotional and FF-ish tunes into the movie too, and not just rock songs for action scenes.