Originally Posted by
ijgordon
If you purchased an I fare with an underlying, say, Q coach fare, they know exactly what the difference is embedded in the fare. They don’t even need to look at the coach fare on the rebooked flight.
It’s usually a fixed up-charge regardless of the coach fare, if you’re booking in I inventory. If you are booking in D, then the up-charge is higher.
Well, sort of
While domestic FC fares are often driven by underlying coach class inventory due to coach fare basis codes and dual inventory checks, there can also be cases where the lower FC fare buckets are empty on a flight and it is actually being driven by FC bucket inventory availability. But no airline is going to go back in time and look at the specific fare bucket inventory that was available on a flight when you purchased your fare (most likely, they don't even have that info available).