Whenever you upgrade, we will automatically look for templates that have changed and see if you customized those templates. If you have, we will automatically try to merge the changes in. If this succeeds, you won't have anything else to do (in most cases). If it fails, your customized template won't be updated--like in vBulletin 3--and you'll have to apply the changes yourself. However, that process is improved as well. More on that later.
Roughly, the merging process goes like this:
[*]Find the differences between the old default and the new default. ("old-new")
[*]Find the differences between the old default and your customized version. ("old-custom")
[*]Start walking through the template. If we find a changed spot in old-new but it's not changed in old-custom, use the old-new version. If we find a change in old-custom but not old-new, use the old-custom version. If neither changed, use either. If both changed, that's a conflict and we can't do the merge.
Most of this happens behind the scenes. However, you'll see some changes to the "Find Update Templates" screen:
The "View Highlighted Changes" link will take you to this page:
This lets you view the 3-way merge results visually. The yellow lines indicate areas changed by merging, while the red blocks indicate conflicts. The conflicting areas show you the values in all 3 versions of the template (old, new, customized) to let you choose how to resolve it.
Of course, trying to do the edit from this page would be a challenge, so if you click the "Edit Merged Text" link, you'll be taken to a normal looking template editor:
However, the text that you're editing is slightly different from the norm. Any unchanged or merged areas are as you'd expect, but the output from a conflict is very different. All 3 possible values for a conflict will be shown, so you can resolve it correct. And don't worry, if you try to save it while there's still a conflict, it will warn you.
It should be noted that the automatic merging is pretty conservative. "Adjacent updates" (when a customized version adds a line immediately after a line that was changed) will trigger a conflict. In some cases, it'd be ok for this merge to go through, but there are other situations where things would break if the change were applied. Regardless, the new conflict management system should allow you to resolve the problem more quickly.