I updated my main forum to 3.8.5 - everything went smooth, and all of my modifications still work. The only problem I had, was figuring out what templates I needed to edit to get the "mark forums read" to work. With some help at vbulletin.com, I finally got the templates edited and everything is working now.
We've also upgraded to 3.8.5 with no issues, we will be going to vb4 when I'm happy with it on our test server, although we only use the forums as we don't have any need for the other bits as we use vbadvanced which is perfect for us.
upgraded 2 of my forums one from 3.7.6 PL1 to 3.8.5 and other from 3.8.4 PL1 to 3.8.5 no probs with either - all went smoothly helped one client of mine upgrade his from 3.5.4 to 3.8.5 then to 4.02 PL1 with no probs either!