I have seen the GA come on board after I've settled in my J seat, near the end of the boarding process, with new J BPs in hand for Y pax. They are promptly retrieved and brought up front.
I have also seen the SD, without the GA, pluck pax from Y and bring them up to J on near-full Y/near-empty J flight (320/321 aircraft). This happens often on transcon redeyes.
Having flown other *A airlines, I have also been the recipient of upgrades after seated in Y.
Last edited by tcook052; Nov 16, 2017 at 2:59 pm
Reason: off topic