title: Future-proofing a Magento XML override
body: <p>I'm aware of the Magento best-practices pattern that says that if I want to modify one of Magento's core files, I should copy the file (and matching directory tree structure) from /app/code/core into /app/code/local and modify the copy so that it overrides the original. I need to do that with one of Magento's core XML files - I've modified the file, and I want to make sure that it doesn't get overwritten later when I upgrade Magento.</p>
  
  <p>Right now, I have a modified version /app/code/core/Mage/Page/etc/config.xml in place - I edited it to add in a specific page layout that I need for my site. Now I want to move my modification out of the core files. Do I recreate the entire XML file in /app/code/local/Mage/Page/etc/config.xml, or do I only need to put my additions and changes into a local override file?</p>
  
  <p>If it's the latter, can someone explain the structure of this to me please?</p>