So I'm thinking, when XF is available, I will create a folder called /xf under site root and run the importer to get all the post/user/thread into a new database.
When everything is working, just rename that folder to /forum
That's the best way to do it. Move your /forum/ to /forum_old/, add an index.htm in a new /forum/ saying "We are moving", while you install the new forum on /new_xenforo/ and do what you have to do. When all is done, rename /forum/ and rename /new_xenforo/ to /forum/. Voila.
Also schedule in time that it takes you to do the import, test things out, configure your site, etc. Before you switch.
Testing a new product means you need to .. test it. Just throwing site live without having some select community users figure out the import has issues, might not be the smartest choice