The team warns against editing core files, because it makes upgrading more difficult, especially if you don't know what you're doing. You need to bring over the new code, eliminate any outdated code, and make sure your own customizations stay in place. Note that merge programs do all of this it's just more files you have to check (beyond your templates).

I would never consider any of the language files to be "core," because by their nature they are meant to be changed. They of course tie into core files, but it's very easy to spot differences, since they're just a bunch of defines.

Plus, all the language files in the catalog work with overrides. So if you edited YOUR_ROOT/includes/languages/english.php, just copy that file to YOUR_ROOT/includes/languages/MY_TEMPLATE/english.php.

This still does not escape the need to upgrade. Moving it to a custom folder will allow you to safely overwrite the original english.php, however now you have to compare that file to the one in your custom folder, adding whatever new defines exist.
Quote Originally Posted by icklebits
Yup, you are right, first time upgrader with very cold feet!
The only way to get over it is to just do it. Remember, you aren't actually upgrading your live site; you upgrade a copy, then test your work locally, or set up a sandbox image of your site to test it. That way you can test without risking your live site, and only push it to the live site once everything is working smoothly. If everything goes to hell, you can wipe it out and start over.