I have an ex-ARN trip to SFO next year in WT, booked into fare class S. It looks like I can upgrade to WT+ with cash but MMB tells me I have to call up for pricing. Is there a way I can work out what the upgrade would cost - is it a function of the difference in fares?
For a rough guide, price the new itinerary using ITA. Subtract the amount that you originally paid. Add the change fee.