xliioper

There are 5 fares classes for D1 (also used for DL FC fares) -- J/C/D/I/Z. There specifically has to be availability in the lowest fare class (Z) on the flight in order to apply a GUC and upgrade immediately from a Premium Select fare. If there is currently no inventory in Z class on the flight, you will have to waitlist and hope that it opens up at some point. If there are currently seats for sale in Z class on flight, that means there is Z inventory availability. Sometimes, there can be Z inventory open on a flight, but no Z fares available for purchase (due to fare rule requirements on Z fares like advance purchase). ExpertFlyer can show class inventory availability on a flight regardless of what is actually available for purchase.