Well, it is taking longer than I expected, but it's working. Yesterday was a whole day of struggling with unforeseen problems.
The biggest problem is that I warp my threads in pairs. There are 20 slots in the tension box reeds and 39 threads per section. This means that all but one thread are doubled up, allowing them to twist around themselves. It's not usually a problem because they're not really twisted. I guess that for the first half of the beam they're twisting in one direction and going the other direction for the rest of the beam or something. When I'm weaving a beam, the action of opening sheds keeps the twisted threads for tangling and by the time I reach the end of the beam they're not twisted any more.
Well, with this operation, there are not sheds to prevent tangles. And so I transferred 20-30 yards of thread yesterday, 6-12 inches at a time. I wind a little, stop and untangle twists, and wind a little more. This continued late into the night and started up again early this morning. No time for tea today. I've got to have this operation done today because the studio, including all 3 looms, gets broken down and packed starting tomorrow.
But here's the final result: acceptably even amounts of yarn on the beam with the extra yarn saved on the spare loom.