The same thing happened to me over the summer where I received an open ecredit and could not apply it to a booking on DL's website. I called and what the rep ended up doing is refunding the amount of the ecredit to my credit card. If you don't like that option, you can also ask them to refare the ticket into FC and see how much that would cost. I am pretty sure you can use an ecredit for the refare.