ACTUALLY my personal opinion is something like this is way beyond the scope of the support of this software. We make no claims to be able to switch integrations.
I think Don has given you a general guideline to follow.
I will add the easiest thing to do is write an import script to import your Photopost user's into UBBThreads. You can electively choose to hand edit your users to the Threads usertable. You would then not need to do anything extra except write the Photopost user data over to Threads. Since the UBBThreads passwords are also md5 and you will be importing members as the same userif/username there is not a conversion other than switching the integration type afterward.