Yes, the portal, side columns and gallery forums are all totally optional, you don't need to enable those.
As for upgrading. Basically, what you'd do is download 6.7.3 of UBB.classic and upgrade to that version. Then you'd download the latest UBB.threads and run the import script that will import your data into it.
You'd need to make sure that your server has PHP 4.3+ and also MySQL 4 or 5 installed.
We have installation/upgrade services available if you need someone to do the upgrade for you.
