This is a change I do not like, although if lots of people use SDC as a way to purchase cheap 6 am flights and then move to more expensive flights at desirable times, I can understand that DL wouldn't "like" this net movement of passengers which would also take later seats out of inventory for use in IROPs. I wonder whether there could be a better way to rewrite the rules to discourage this behavior while continuing to permit SDCs to later flights under reasonable conditions.

Is the lesson to be learned that we should always buy a ticket for the latest feasible flight and then try to move it forward by SDC if we decide we'd like to travel earlier?

Do I remember correctly that elites (maybe GM+) can SDS to a later flight while nonelites cannot, even for a fee? OTOH, SDS for a later flight is a risk that I generally wouldn't want to take.