I wound 40,000 feet down to two 400' and one 200' for a feature that got a special deal from Fuji. Most of the feature was hand-held, so the need to convert. The trick for us was to roll 600' across (we measured with a frame counter on proper camera cores), can and bag the remaining 400'. Roll back another 400', can and bag. Then we took a few feet and twin checked, placing back in the original can, processed to assure no fogging. Then rolled back the final 200', cut a piece off the front that would get exposed to light, processed for scratch test. We figured that part was handled the most and if we did anything to damage the film, it would show here. When we canned the rolls we labeled with the lot number etc. from the original roll. It was a lot of work, but worked great. All the safety checks we built in became unnecessary.
If you count real well in the dark, you can do this.