You need someone to look through the e-Ticket history section of the PNR. They should be able to see when the ticket was exchanged and where the value for the fare difference went as part of the exchange. It should indicate if it was refunded to a credit or to the original form of payment. If there isn't, the value is stuck in limbo and probably needs to be teased out by the refunds department. As others have indicated, you may have better luck with the DOT as it will ensure getting the right people to look at it as opposed to phone lottery if you get lucky to find an agent who knows how to review eTicket history.