I never paid the difference to a higher class on the return trip: the ticket agent always told me that I have to pay the difference for the whole RT though I had already flown the outbound. So it's normally possible to pay the difference, but for what a price ("worthless" outbound)!

Asking the airline never hurts. Good luck.

[This message has been edited by LH738 (edited 08-17-2002).]