Also when your itinerary mixed with code-shared flights, it will say ineligible when it is not the case. I book alot ORD-NRT-HKG mix UA/NH flight and I always can applies SWU on the ORD-NRT segment. Clearing the upgrade is a totally different topic though But last time my upgrade ORD-NRT was cleared, and get op up to C on NRT-HKG NH flight^