Question about rebooking a flight on Delta, after cancelling.

[SeaSpray]

DH and I cancelled the trip to WDW we were supposed to be on right now. We cancelled our flights with Delta but will be charged $50 per ticket for the changes. Can anyone tell me what's involved with rebooking? The first time, I booked online and paid with a credit card, but this time I will be applying the balance of our refund to new tickets. Is there a way to do this online or do I need to call Delta?? I know I can call and find all of this out but I was wondering if anyone here knew the procedure. Thanks!