KE does allow you to buy up at the airport, but not for cheap. I used to regularly ask what the Y to J buy-up costs would be when flying KE (granted I haven't done so now in a couple of years). I was always quoted the full fare difference no matter how empty or full the J-cabin was.

As such, you may be able to buy-up for $500-1000 for a shorter international route depending on your original fare class and which fares they are ticketing in the J-cabin; but I would be shocked if you found a buy up in that range for a trans-pac flight.