Originally Posted by baliktad

Make a pretend booking on alaskaair.com for the legs you care about (ATL-SEA and SEA-GEG) and look for the U next to the Full Flex fare in schedule view. If it shows up, there's U availability. If it doesn't, AS hasn't yet released the seats for upgrade. The number of available F seats really doesn't have any bearing on how many U seats there are - that's entirely up to AS Revenue Management.

You can keep checking every day for U availability, or you can subscribe to a paid service like ExpertFlyer which will search constantly and automatically alert you when it finds availability.