Followup: NO, you can't. Worse yet, the agents don't know it, so they tell you that you CAN, and you potentially lock yourself out of the J upgrade queue. You cannot manually switch upgrade queues once you're confirmed - and you need Ticketing to be open, which means that if you're on the wrong side of the world, you're screwed until about 7am Eastern.

Basically, you must pick the right queue right away. I suppose it's fair, but I don't like it. Also, I wish agents were better informed about what the process is. Oh, and since you upgrade into N space, when they look at what's wrong, they have no clue because unlike R space, where they can clearly tell you've upgraded, N space makes it look like you have a cheap revenue PY ticket, so they don't realise they need to remove your upgrade and switch you to the J queue. Sigh.