When doing installs/upgrades I normally ask where they want it installed, in the same directory or a new directory. Most choose a new directory so they can leave the old one fully intact. It makes it easier for them to go back and match their old styles, settings, etc.
The only real benefit in installing into the same directory is that if you have old links to the main ubbthreads.php script in that directory, they will still work without a redirect. Everything else, the process is pretty much the same. You have to keep the redirect scripts in place.
And this only applies to UBB.threads. Any UBB.classic customers that have upgraded have to install into a new directory. Well, in theory they could install into the old ubb directory but that just gets messy.
