You can't just make your forum read-only for say 24 hours (so folks could still access things), and do a database backup after you make things read-only .... then do an import of the database, then bring the new forum back up? If you did it that way, you could make the cut-over after the import and make down-time almost zero (you'd have to make sure your configurations were correct with the new forum, install necessary add-ons, set up redirection, and so forth). This would be pretty easy especially if everything was on the same server. You could even cut-over using the same URL.