The issues you need to be aware of whenever you upgrade is to not have to re-do your modifications. There is no backup system for the templates. Even the built-in database backup system is not very good. You should have a couple of cron jobs to take care of regular backups of the database and the folder/file system. Then you have to manually save the templates, at least the ones you have modified (they're easy to spot - they're the red ones in the list) to a text file. I use "
Beyond Compare" to compare files I have modified to the new ones so I can edit in the new modifications. The template update/edit system is also good for making sure your mods don't get lost.
It's a time consuming task and you should know what you're doing. Upgrading vBSEO and SiteMap is much easier. That only takes minutes.
The vB team is very close to a new release version - going to 3.7. Unfortunately, that means there will very likely be a 3.7.1 or so in the following few months. Because of that, I would recommend upgrading to 3.6.8 PL2 now and then plan on another upgrade to 3.7.x in 6 - 8 months. The current release is very nice and the new, unencrypted version of vBSEO and the more versatile sitemap are excellent.
Jim