Any Guide to Fix Skin after 2.1.x upgrade

I’ve read the upgrade from 2.1.0 screws up custom skins. Is there a thread or sticky or tech doc anywhere that outlines the causes so we know what we need to modify? Or can someone throw me a bone as far as the best procedure to take in getting a custom skin to work again on 2.1.x?

After upgrade you can view what the changes were… I just open up an editor on the new version and cut/paste my custom modifications back into the updated template