I’m in LHR hoping to SDC to an earlier flight, 928, tomorrow. 928 is wide open in both PZ and W, so I thought switching my GPU from the later flight would work. But two agents have already said there is nothing they can do because LHR-YVR doesn’t have W inventory currently. We already have W space ORD-YVR and were not looking to change that flight

Is it the case that you have to have through fare inventory to change even one segment on an itinerary with a connection?