Just to finalize the thread (Mods, please close,)
It took me about 15 minutes at the Nong Hoi DLT.
I went up to the information window with blue book and passport, the agent filled out some papers, then had me make copies (next door) of the main page and current visa stamp, then sent me to another window. Only one person ahead of me for license plate stuff just finishing up, and so I was seen in two minutes.
I sat with that agent for 10 minutes while she did some more paperwork, paid 140 Baht for a new tax stamp, and left. I'll come back in 30 days to pick up a license plate.
Couldn't have been much easier.