I called this evening. Asked to buy the 24 OTUP using the Beyond 2-for-1 deal. I asked if he could directly swap out 24 points on an existing reservation. Sure, no problem, he said. He took payment via my DGC, put me on hold for a few mins, and came back to confirm he had secured the 24 points ($240 total cost) and already returned the 24 points to my '24 UY (which I can still bank). Total call time (including the all 5's survey at the end): 12m18s.
I have no idea what went so wrong that after 7 days my original attempt to buy OTUP couldn't be processed, but this was quick and painless. Thank you for the prompting to try it again!