I also had a similar situation. Unfortunately, a DL/AF award ticket booked by NW with NW miles leaves you in a kind of limbo in these situations. NW won't be able to help you if it cannot find PerkSaver availability on an alternate flight. They will direct you back to DL, who may be willing to rebook you on another flight at no extra charge, but that will be entirely at their discretion.

Who's schedule change created the problem? AF or DL?