What I would do is load both ROMs into a hex editor like HxD, then do "file compare". Some revisions can be pretty substantial, others not. For example, I just did a translation of Mahjong for the Famicom, and there are two revisions. When I did a file compare, I discovered that literally the only difference was that the number 8 looks different, so my patch works for both.
In some cases, code can be moved around, in which case creating a patch that works on the other ROM could be a bit of a task. It depends, basically.