Help needed in converting mod

Hi all!
I'd need your help in a -I think- not too difficult problem.
There's Renaissance Total War, of which the publicly available version is not compatible with M2TW v1.2. But I'd like to make it that, so that our fans don't have to keep a (huge) copy of M2TW 1.1 just to play RenTW.
I myself am a modder, and generally know what things in text files do, but I don't know what causes the incompatibility of mods written for 1.1 with 1.2.
So, could someone please tell me what needs to be modded in order to make it compatible? Or at least could you point me somewhere where I can read about it?
Thanks in advance for any help.