No problem at all, I tried that a couple of times. Just call MPC or reservation line, to tell them you want to do an " Up-Fare" from M to K class. However that you will not be able to do OLCI because you need to go to airport to pay the fare difference, they will not accept credit card on the phone. If you want to do OLCI you will need to go local ticketing office to pay the fare difference.