It may not be fair, but that is the way it works for most software. I use the perl version and do not intend in the very near future to switch to the PHP version (for a variety of reasons) so the balance of my upgrade license will be of little use. But I can not say that I expect any kind of prorated amount on the original license either.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.