Our forum, which is on vB 3.8, currently sits in the root directory. We are ready to make the move over to XF 1.3. Once the move is complete, we want to keep the vB version on the server for a bit, hidden and password protected in case we need to reference something.
I think it would be best to move the vB forum to a different directory, install XF, import the database, and then start customizing and tweaking features while being live.
Another option I would assume would work, is that we move vB to a different directory and keep it live. Then install XF in the root directory, import the database, tweak and customize, then when we're ready to go live, import the database again to update users and posts (is that even possible?), and then ... well, be live.