so I flew TIJ-PEK on July 5th. It seems that AM handles HU's check-in, etc. Check-in agent told me that AM has no control on class changes, thus upgrades to J were not possible at check-in. She said upgrades to J are handled once you board.

So we boarded and asked one of the FAs at the door about a cash upgrade to J, she said she'd talk to the cabin manager and get back to us. A few minutes later, she comes to our seat to inform us that an upgrade would cost 8,300 yuan, or approx $1250USD per person. My friend and I happily declined since we both thought that price was too much considering we'd be getting a economy meal and the miles earning class of the economy booking class we originally booked.

I hope more data points can be added about this thread just to give people an idea.