If the hotel wants to avoid accepting the corporate rate during certain periods, it needs to set up its reservation system not to offer corporate rates on such dates. Otherwise, once the reservation is confirmed, it should be viewed as a binding contract on both sides.

However, this won't help the OP to have a bed in which to sleep during a period when a foreign city is likely to have no hotel solace available.

Assuming that this is a business trip, turn the issue over to corporate travel. It would seem to be step one in getting an exception for a higher rate.