The refund should be the difference between what you paid and the cheapest coach fare that was available when you purchased your tickets (not the walkup coach fare available on the day of travel). DL should also give you some customer service gesture, probably in the form of RDMs.

Do you have status? What is your fare class? The fact that you don't have a seat assignment doesn't necessarily mean that you will be the one to be downgraded if a downgrade must happen. However, just in case, it's wise to check in early and arrive at the gate easily to speak to the GA.

BTW, was there a change of aircraft? Did you ever have a seat assignment for the flight?