Read Aaron Hawks' blog post for the original idea for this gist.
While this process should theoretically work to upgrade the core/default files that come with Magento from any old version to the most recent version (as long as they're in the same distribution, i.e., Community to Community or Enterprise to Enterprise), you should always refer to Magento's release notes and upgrade paths for the version you're upgrading to for full details and work flow. This is the process I used to go from Magento EE 1.12.0.2 to Magento EE 1.13.0.1 directly ( skipping over 1.13.0.0 due to the instructions outlined in this Magento article and release notes ).