It would be pretty easy to rebuild using pgc edit. You'd just need to rip all of the streams using DVD Decryptor, or something similar, and then remux using your new ac3 file. You could create a very simple DVD with just the film and (as long as you've recreated your chapters) you can replace the main movie (VOBs) in your original dvd with the one from your newly created version, using pgcedit. This way you don't have to do any work to recreate the menus or programming of your original DVD for the newer version.

If you share the ac3, I could redo this within a day and repost to a.b.starwars.