Reviving an older thread as the 'upgrading on AC' thread seems to be closed.

I have a friend who purchased YUL-YVR-OGG return. J on the outbound and Y on the return (Fare code GNA5A2CO)

He was offered to bid on the YVR-YUL leg on the return, but would prefer just buying an upgrade on that leg. Does anyone have experience upfaring one leg of a booking? Flight shows J9/R5.

Thank you in advance