I'm not sure I understand the confusion here. If Delta cancels the flight, OP will have two options. A full refund of miles and cash (taxes) or a reroute at no additional cost (assuming one exists). Those are the options and have always been the options.

A fare difference has never been a requirement to be rebooked if Delta cancels the flight.

If, in the other hand, the OP wishes to make a change to their itinerary, then the new current rules come into play. No change fee but a difference in fare applies.