I have a companion certificate from my delta skymiles card that I am trying to use. It appears that it is blacking out a lot of my return flight options on Sunday. I think this is because the certificate only works on L, U, T classes of service.

Originally I just figured that I was SOL but I recently noticed that an extra fight appeared when I re-tried the search. My thought is that Delta "re-classed" the new flight so it became eligible for my certificate.

Does anyone know how often they "re-class" the economy cabin? If so should I continue waiting to see if a flight that fits my schedule "re-classes"?

Thanks so much in advance for any guidance.