[p0lar_bear]

If you've got source file edits as well, make sure you know what you edited and where; if it's a downloaded mod, just read the readme again to refresh yourself on what was changed.

Just upgrade the board as normal. VBulletin has a nifty feature that will tell you if any templates that you customized were changed since the last version right on the ACP homepage, and will let you view the changed between the stock template and your customized one; kind of like looking at diffs in a wiki article. What I tend to do is look to see if anything was added or removed between the two versions. If something was added to the master template, I add it to my template. If something was removed, remove it. If something was changed, I assess the changes it points out and see if my customizations would break functionality.

Then after template edits, go onto the main board and check to see if anything's broken. If any plugins aren't working, try to figure out what would break them, or just reinstall them.

Finally, redo any source edits you took note of in the first step.