Originally Posted by
DaveTO
You will keep your seat assignment unless your upgrade goes through.
However, if you change your mind and refuse the upgrade at the airport (?!), your seats could be picked up by any other traveller and you'd be on your own.
I think that one also has to pay the "fare difference" if the original booking fare is no longer available. For example, one is booked in H but only Y is available when one changes the mind, one must pay the fare difference between H and Y. Changing mind can be expensive. It's more than just losing your originally assigned seat.