I'd call IHG and complain. Especially if you have any status, including credit card status, they will likely throw a meaningful number of points at you. Take them. I'm sure IHG won't be able to do anything about actually gettting you a room at this property now. I agree with azepine that your best bet is to stay outside of Springdale: it's not that far, and tends to be a much better value on both points and cash.

BTW, Hyatt and some other chains have generally been good to me when holding reservations at a hotel that changes flags. They usually negotiate with the property to honor reservations -- including award reservations. A bit surprised that IHG did this to you here.