A couple of months ago I was on UA 860 GIG-GRU-IAD, a one-stop direct flight with the same plane and same flight number. The last day or two of the trip leading up to boarding in GIG was a mess, so I felt lucky to make the flight -- not much opportunity to fine tune the arrangements. I flew in Y on a W fare, and during the 2+ plus hours between the segments at GRU, I verified there were available C seats. I tried to upgrade with a GPU but was told that there was no way the system would allow it because I was a "transit passenger," having joined the flight in GIG. The flight went out with 8 empty C seats.