I asked whether desks can reprice for same day flight changes; it seems this right has been taken from desks and must go through Global Contact Centre https://www.flyertalk.com/forum/cath...l#post35970616

I imagine the same goes for same day class upgrades - and your ticket must be capable of being reissued. So even if you are willing to stump up the fare difference on the day, the answer may still be no.