Delta does indeed consider D1 Suites and D1 to be different products. Unless policy has been thrown out, there is a policy allowing rebooking jn these cases. In fact there can be additional charges for D1S aircraft over D1.

Someone mentioned QR. They by policy do allow rebooking if booked QSuites and a non Q-Suite is subbed in. Their policy is a lot more generous. It basically treats it like a flight cancellation (in terms of what you're allowed for rebooking).