Originally Posted by Guy Betsy
Or else just ask at the Ticketing counter at the airport about the upgrade. There really isn't any set price.
I've done that one many a time, and on various sub-classes (but all mileage-earning of course). The price has
always been THB 5000.
To the OP, check-in as normal and ask to buy the u/g. If your fare is valid (ie milage earning) and of course there's a seat free, they'll check you in then send you to the CX office on the far wall adjacent to the CX counters to pay/pick up BP. All fairly effortless.