I think I am gonna attempt this. The problem is I have made quite some changes to the core codebase myself to address some features I need that's not provided. So my game plan is to just diff through each files one-by-one and hope for the best! Anyone got any helpful tips?

Actually, I do have a question. There shouldn't be any DB schema changes/addition from 1.1.5 to 1.2.2, right?

Well, now you will understand why we recommend not doing that! I think you will be overwhelmed with differences. As for schema changes, most likely (without actually looking) there are changes. Nothing that the upgrade process should not handle, though.

Maybe you should consider what features you added compared to what zenphoto does in its current form.