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.