I am trying to use a BXP1 on JFK to SFO in Business Class. I would like to book an Economy ticket then upgrade to Business Class. I called the agents and asked which fare class the upgrade comes out of; she said U. The help above says C. C makes more sense and I see more availability on ExpertFlyer. Does anyone know for a fact which is the actual class needed for a BXP1 for the upgraded class in Business on the JFK to SFO/LAX route?