DL seems to be insanely tight/stingy about advance upgrades on inter-asia flights to NRT. No idea why. I'd keep calling/checking, and if you don't clear in advance make sure you request to be put on the standby list at the airport when you check in, since if there are available seats you should, theoretically, clear at the gate since you are using an SWU. The only concern is that many agents don't seem to be trained/informed about how to correctly process the airport standby upgrades, so I'm not sure what will happen in BKK. The ticket counter agent has to put you on the airport standby list, and then the gate agent has to clear you into BE if there are any seats open... and you should be cleared BEFORE any non-revs get seats in BE and not have to pay any miles or anything, assuming you are using a SWU and its already in the system... but I'd take a copy of the cert number and redemption code anyway.