Well mods that are still designed for 1.2 won't work, but there are actually more mods designed for 1.5 than 1.2 now anyway, even things like RTR which still have a 1.2 version also have one for 1.5. Exactly what were you trying to install?

Anyway I don't think you can go back, even if you could persuade the 1.2 patch to install onto it that wouldn't necessarily undo all the changes made by 1.5 so you could still be missing some earlier versions of files.

P.S. love the user name !