From what I understand, IF you clear before the flight goes to the departure management list, it is FIFO. If it is on the DM list, it is based on these factors:
1. Status
2. Fare paid
3. Time added to DM

With only two unassigned C seats, it is possible C is actually full and you will only clear if there are no shows. Do you know what the loads are as of now? What date are you traveling?

Cannot speak from experience of upgrading this flight, but when I asked if the HKG-SFO flight was a dangerous one to have to "waitlist" for an upgrade with, everyone on this board says yes because it has a lot of paid traffic and there are a lot of elites on the route.