You are free to do it whenever you wish to. Airlines do not audit ticket coupon usage except upon a complaint by a passenger.

The reason you can do this at any time is largely because the airline retains rights to do things like change seat assignments, cancel flights, re-route passengers, etc. As a passenger, you also retain the right to make changes and the carrier has no penalty adjudication ability any more than you can bill the carrier a $25 seat change penalty.

Their is no legal concept or theory, as a result, that the carrier can utilize against you. That is why they do not and will not enforce it.

For instance, I was in White Plains on 9/11 when commercial aviation shut down for 4 days. I rented a car to get back to Dallas and threw away my return AA ticket. I dropped one person in Memphis and 2 people in Little Rock on the way.

What you do when you get to where you're going is not within the control of the carrier no matter what anyone at Delta tells you.