Absolutely!! However, you can't just change the flight with a Ding!
If you get a Ding! you like then go through the booking process except for payment, then open a new window and cancel your old flight, then use the travel credits to pay for the Ding! flight.
That's why it is important to book two one way trips rather than a round trip. Sometimes one leg of a trip will have a DING! fare available, and the other not, or rates have gone up on one leg or the other. If you have to cancel the whole trip rather than one leg, the savings may not be worth it. Just a nice habit to get into, booking two one ways.