Officially you're only eligible for a cash (credit card) refund if you're on the exact same flight and the exact same fare bucket or if you booked a fully refundable fare.
Every fare has a letter code associated with it, if the letter code is the same then you're eligible for a cash/cc refund. That will never be the case if you're going to a DING fare but might be the case if you booked an internet special fare and a lower internet fare comes out.
SW is one of the few airlines to allow a full credit, no change fee, and allows you to use your credit for a different passenger. Let's not give them a reason to change this generous policy by making multiple requests for exceptions. Nothing wrong with asking once or maybe twice but after that either use your credit or find someone else who can.