Even if you install into the same directory, you'd still need to keep the redirects in place. The main reason for this is the old versions called things directly, like showflat.php, showthreaded.php, etc. In version 7 everything goes through ubbthreads.php. So, if the redirects are removed and someone has an old bookmark to showthreaded.php they'd just end up with a 404 not found.
