Thank you both for quick replies! I did some further searching and found two threads that suggest UBBThreads 6.5.5 -> 7.5.5 or 7.5.6 is the best approach.
Ruben, you are correct that I had to spin up a very old version of xampp. The earlier versions of 7.x are incompatible with mysql 5.5+. Running this test on xampp 1.5.2 with php 5.1.2 and mysql 5.0.20. (randomly picked this version from sourceforge as its build date was around 2006). I usually run UniserverZ but it doesn't even support mysql that old. Thread suggesting success with 7.5.6Thread encouraging direct import to 7.5.5
I am attempting a direct import from 6.5.5 to 7.5.6 currently. It's in the middle of importing topics, and I can say this time that the user_id column in the _posts table is updating correctly.
I will report back when the upgrade finishes later tonight.