yes, you should be able to upgrade from 3.1 to the latest version, just upload the setup files and run them to upgrade your db, then upload the new forum files
This is ony true if you have not modified your code. The best part of SNitz is the open source, and I have modified it massive extent for special sites. That means I can't "upgrade" as it would actually be a downgrade.
TIP
Put your add-ons and mods into <!--include files--> so it's just a simple line to change. I do that now with 2 newest sites and it really helps.