The current method for transferring content from a UBB.classic board to a UBB.threads board can be found in the Admin Guide.

Basically, you will need to upgrade your UBB.classic board to 6.7.2 first. (This is actually a 2-step process: first upgrade to 5.47e, then to 6.7.2.) Then use the Import/Export feature to export the data (members, categories, forums, and topics/posts). Copy those data files to the /import directory of your UBB.threads installation. Then place a copy of the UBB.classic YAML file there. (YAML page). Then run the Import Data feature in UBB.threads. That will do it.

If you cannot get the UBB.classic 5.47e files, put in a support ticket and we can provide them.