It is a little more than just upgrading your forum. You have to run import scripts on your old forums to load the data into the new database. Threads 7 is not just an upgrade, it has been a complete rewrite.
I recommend waiting until the Final Version 7 is released. I've only had it on my public board for a couple of weeks (I'm a member of the beta team).
If you decide you want to test importing, you should setup a 2nd MySQL database and test outside your main forum directory. That way if you mess up, you don't break your boards.
The Install Guide can be found at
http://ubbcentral.com/support/docs/ubbthreads/ubbt7install.pdf
