Recently booked a trip from New York to Madrid on Delta stock. Outbound non-stop on Delta, upgraded with GUC. The return is on KLM operated flights (Delta marketed) via AMS. Agent said that inventory was not available on KLM to upgrade on the return . If I search on Expertflyer, both individual segments show Z inventory (9). But the entire trip with married segments shows no availability. Is it true that availability must be available for the entire return itinerary? Or can the married segments be "unmarried" so that I can upgrade? All I really care about is AMS to JFK. There are a few other threads on this topic, but I couldn't find a definitive answer. Thanks.

Dave