Sometimes there are necessary changes made to the default skin for new features or to fix bugs. You would probably be better off to do a diff between your skin and the original, and then recreate those changes with the updated skins.
Using older skins with a newer codebase is risky, and there would be no way to support it.
If you have your skin in its own folder, and did not modify the default skin directly, then you can always just put that folder back in and check for changes.