Yes! You can get a credit for a future flight valid for a year from the date you originally bought this ticket. The new reservation that you use the credit on must be booked and flown completely by the funds expiration date.
To get the cheaper price just click change reservation on southwest.com or the app and click on the cheaper flight it will show you how much credit you will have if you complete the change. Once you click book flight the credit will be held under your current confirmation number and the credit can only be used by the currently ticketed passenger (credit is non transferable).
-Blake
