Just a name change I believe to distinguish between UBB Threads (PHP) and UBB Classic (Perl). Upgrading would be a good bet since the newer versions also resolved many of the security issues of past versions, not to mention some of the features such as Recent Visitors (replacing Who's Online), PM, Content Islands (site intergration), Sticky...
Since there isn't too many 6.5 hacks now you'll still be able to use all 6.4 as well as previous version hacks with some modification to the hack files.
