You would not import anything. You would simply upgrade your forum reviewpost and classifieds to the latest.
If your trying to install a clean forum and want to reinstall/move your reviewpost classifieds to that forum your going to have issues since no users exist on that new forum therefore you really if your installing fresh install everything fresh.
I guess I dont really understand why your talking about a clean install when all you need to do is upgrade. If your trying to transfer everything to a new site and discontinue the old site then thats what you would do which means moving all the data not starting fresh.
Those are good points Chuck, so I can see how it won't work.
Over the few years I've been running vB I've installed and removed many mods and styles. I guess I just wanted to start over with a nice clean site and database. The site has been growing and I want to avoid future problems.