I would fill out a support ticket and request the UBB 6.2.1 full install zip. Inculde a URL to this topic, and explain that you need it for the transfer. Assuming your license was vaild at the time 6.2.1 was released (I'm guessing so since you have it loaded) it shouldn't be a problem.

It might be safe to do it with the beta, but I would rather get the full zip and do exact versions than take the chance of having a problem using a different version, even if it is BR1 vs the release.

I believe that you are looking for ubb621-release-full or ubb621-release-upgrade. You might as well get the full to be on the safe side.