If you have not flown any segments, yet, go through the process to "change" the reservation. If the same fare type originally purchased, i.e. WGA, is still available for your original flight, select the flight and continue, till you get to the "Air Itinerary and Pricing" page. At the bottom of that page, there will be a fare summary; it will show the original fare, as well as any difference in cost, if making a change. Do not go any further, unless you really intend on changing your flight(s).

Normally, promo codes state that any changes to your itinerary may void the offer on that flight.