Import new and old projects into a new project, then compare each corresponding comp in old and new, deleting the older version once you've determined it.
I love it! Simple, but effective. I should have thought of that.
If you're having trouble spotting differences in the old and new versions, you can use this procedure:
To avoid confusion, I'll call your older comps CompA and your newer comps CompB.
Drag CompA onto the "New Comp" icon in the project window. This will create a new temporary comp that contains the nested CompA.
Drag CompB into your temporary new comp. Ensure it aligned frame-for-frame with CompA.
Set the upper layer (CompB) to Difference mode.
Now when you play through the comp you'll see colour differences at any point where CompA and CompB are not identical.