When do you turn your board offline?... Some forums turn the forum offline over the smallest things such as revamps and changing of forum icons, new forum banner, a new service or a permissions problem.
To me i think alot of the time turning the forum offline is unnecessary and will only hurt the activity of your forum.
As far as a revamp,
Anyone who is making a forum style should do it on a test board and when its all ready to go, just copy and paste or upload (depeding on the software) it to the real forum.
When I upgraded from 3.0.0 to 3.0.1 I didn't take mine offline. It was off for about a minute when my upgrade was complete and I had to delete the install file, but that was all.
We did the automatic update and that updated all the files but the functions.php files as there was some conflicts so i had to download the new version of that file then redo all the edits for that file then upload it.
I do any work on the forum in the early hours (unless it is a major problem). As a UK forum there isn't much happening at 4am so it's when all work, updates, backups and such are carried out. That way there isn't an affect on activity