I'd still check during your trip regarding the return BKK-NRT segment... the system may be showing J0 which made the agent think its all sold, but my guess is that it really isn't all sold out. I saw this occuring regularly with my ICN-NRT leg... J inventory kept going all over place from J0 to J6, and then I finally cleared 1 day before the flight and they upgraded 2 more people at the gate because there were open seats.

And yes inventory looks very screwy when you only look at the one segment... the married seg logic as 3Cforme mentioned really confuses things when trying to figure out inventory. For example using EF, if I put in just BKK-NRT on 4/8 I get J0Y2, but if I put in BKK-HNL through NRT, I see J6I2Y5, which sounds pretty good for the upgrade clearing, and I strongly suspect there are way more than 6 J seats available as well.