Unless you purchased a refundable fare (Anytime, Business Select, Senior, Child/Infant) you can't get a refund. But you do already have Travel Funds for the difference. They are held under the confirmation number for the flight you changed. There is a place for you to apply those funds when purchasing a new flight.
Be aware that the funds are only valid for 1 year from the original date of purchase (not travel date), and that all travel using those funds must be completed by then (not just booked). Also -- very important -- any new travel using those funds will take on the earliest expiration date of any funds applied.
