Partially used ticket has to fare based on what was flown on the outbound. You can't do a straight up new itinerary search and then apply it, because that new itinerary search didn't consider your outbound fare component. You need to go to my trips, pull up the PNR and then "use future flight credit" which will bring you to essentially a change booking search page. If that doesn't work you'll have to call. If you're getting $2000 fare differences where as a new search (before trying to use any credit) is only showing like $200 it is probably a fare combinability issue and forcing you into a higher fare class (less restrictions) to be able to combine with your outbound component. In general if you're using a FFC that is a partial of a round trip, you need to fly to the same general area/global direction as the original round trip to get a fare that combines with the likely discounted outbound fare component (ie if flew US-EU, you would need to rebook EU-US since those discount fares generally allow half-round trip open-jaw/double open jaw combinations with each other).