It's not "allowable" with your current ticket. But if you call, Delta might grant you the change. They would have to completely reticket, and re-calculate the fare, but they may give you the change with no extra charge out of goodwill. It's worth a try. (That said, if you pull up the schedule, and it was CHEAPER than your original, you are, of course, better off cancelling for a refund, and rebooking it yourself.)