I'm not sure you'll be able to change it over. I haven't tried to do that yet. From reading the rules, you should be able to cancel though and take the ticketless travel funds and apply them to a new purchase. Keep in mind that ding fares are not combineable with other fares, so if there's a sale fare that's better than a ding fare you can't book them on one itinerary, you would have to book one way flights to do that. And I have found some some ding fares are cheaper when booking round trip than one way.
Sorry I don't have a more definite answer for you. I haven't tried to change a ding to a regular fare...yet.