Just a comment: a facility declaring ATC Zero does not cause the airspace class to change. If it was Class C before, it remains Class C, just with some published services unavailable. We cannot alter an airspace designation via NOTAM. Airspace definitions are published, and what is published cannot be altered. If the impacted facility is not a 24/7 operation, then the airspace class changes at whatever time it is published to change.
This is something that we even have to explain to the facilities, as many thought the same way and had such language in their ATC Zero plan. If a Class B airport (LAX, SEA, ORD, etc.) goes ATC Zero, the airspace remains Class B, and the facility that assumes control (usually an overlying TRACON) will provide Class B services to the extent possible. Obviously, the TRACON can’t control ground movements.