Simply a data point - any flight to or from NRT has been running heavily booked late this month. UA rez tells me it is due to spring break. I sweated out an upgrade on Monday from SFO-NRT (already had NRT-BKK). I checked in from the 6:20am flight from SAN in order to get as early a DM time as possible (was originally booked on 10:30am flight). I cleared at the gate, as did a dozen or so others - but many did not. I checked on the early SFO-NRT flight but it was overbooked and looking for bump volunteers. The challenge is that you have to be able to clear into coach before you can go on the DM list for the upgrade.

I face the same situation coming home tomorrow. I am booked BKK-SFO as one flight number. It has not cleared. Another FTer who is flying tomorrow is booked on the same flight, but got UA to break it as BKK-NRT-SFO. He has cleared on the BKK sector, but not the NRT sector.