If you hadn't used vouchers, you could cancel and rebook since its within the 24-hour cancellation period. However, I think the vouchers are non-refundable.

Is your ticket in a higher fare class? You may find a kind agent on the phone who might switch you to the IAD flight, especially if there is the same fare class available. IAD and DCA are interchangeable co-terminals in terms of standby and routing.

Standby is the other option. Standby for the 777 flight. A lot of times the GA will give you a seat when you ask and then you can be put on the DM list for upgrades and go from there.