I do not beleive there is a converter for that. I am sure if you pay a developer to write one for you it could be done. However to reiterate what Josh has already stated WHY? That is a backwards move.
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.