I never went from ubb threads v6 to v7.
I went from ubb classic to ubb threads 7.2.
So after you purchase member access I would upgrade to ubbthreads 6.5.5 then goto say ubbthreads 7.2
Watch out for the extra global moderator group. ( in my case all regular moderators became global moderators) But it is a easy fix because there never were Global Mod's in your version
After that you should be able to upgrade to the current v7 version.
Just read the upgrade directions in the install folder at each step.
You could take smaller steps in upgrading but it will take longer.
And hopefully you don't have any mods installed.