Hi,I have a general change fee question.
I know that all major carriers charge around $100 as a change fee plus any applicable fare difference if you want to change your travel dates.My query is-Can you change the return portion of the trip after you have taken the departing flight?And,if so,how do they calculate the fare difference - is it based as a one way fare because they do not have the original departure date to calculate the round trip fare?
Thanks.

Yes, you can. When changing the return portion, the fare difference will usually be $0 as long as you abide by the rules (usually things like day / week + minimum stay restrictions) and if the fare class is still available. When changing the outbound flight, the usual headache causer is the minimum purchase requirement which is no longer an issue for the return.

If your new flights violate the fare rules, the airline will bump you up to the next cheaper round trip fare that you qualify for. They have access to the fares in place when you bought your ticket and will give you the cheapest fare published at that time for your new flights. You will then pay the difference + change fee.

Sorry if that was a bit confusing. Perhaps someone else can explain it better.

Cheers,

It is not based on the one-way fare unless the circumstances of the new return flight don't qualify for any round-trip fare. Actually, they DO use the original departure date when calculating which fare is applicable. Let's say you fly outbound on Thursday and are booked to return the following Tuesday, and you have a rt fare that requires a Saturday night stay. Generally, you can move the return flight forward to as early as the first flight on Sunday (in order to meet the Saturday night stay requirement) or as far out as the fare rule will allow. Assuming there is a seat on the new flight in the original booking class, you will pay the change fee only.