Originally Posted by
fjord
Review the rules for diamonds. Class of service has to be same or higher. I've tried to change from BWI to DCA a couple times, seats were for sale and all, but denied because the class booked was lower than all available classes. I'm DM.
The rules, as published, are the same for Diamonds. Co-terminal is an unpublished benefit.
I believe co-terminal works for SDS if you don't have checked baggage, too.