My reading of the fare rules is that before departure, voluntary changes to the inbound, with no changes to the outbound, are permitted and are repriced using historical fares as of the previous date of ticketing:
CALCULATION OF REISSUES //CHANGES BEFORE OUTBOUND DEPARTURE// WHEN THE FIRST FARE COMPONENT IS CHANGED THE ITINERARY MUST BE REPRICED USING CURRENT FARES IN EFFECT ON THE DATE THE TICKET IS REISSUED. ----- WHEN THERE ARE NO CHANGES TO THE FIRST FARE COMPONENT BUT OTHER FARE COMPONENTS ARE CHANGED THE ITINERARY MUST BE REPRICED USING HISTORICAL FARES IN EFFECT ON THE PREVIOUS TICKETING DATE OR CURRENT FARES IN EFFECT ON THE DATE OF TICKET REISSUE WHICHEVER IS LOWER. -------------------------------------------------- //CHANGES AFTER DEPARTURE// THE ITINERARY MUST BE REPRICED USING HISTORICAL FARES IN EFFECT ON THE PREVIOUS TICKETING DATE.
(Emphasis added)
I haven't tried it, but as I read it, one should be allowed to change the return portion without repricing at current fares.