I was having the same problem, trying to get mileage credit for flights booked in WT+ and upgraded with miles. The system was showing the flights as booked in U and therefore "ineligible". I wasted a lot of time trying to get a live person on the phone, then thought to do a search in this forum. The advice posted by G-BOAG worked like a charm.

Many thanks. BTW, how did you figure this out?