I watched an oversell situation unfold on a busy Friday for NRT-SFO on a 747. The seat map was nearly full, yet they were still selling C and D fares up until ~6 hours before flight time. By the time I got to Narita, they were looking for voluntary downgraders -- repeatedly and urgently. A lot of Y -> C upgraders were downgraded involuntarily (and compensated, and given decent seats in Y).

Since my upgrade never cleared, I didn't get to take part in this game but it was interesting to see that they were still selling those $4k one-way C fares even though seats weren't available.