I was on the phone with IHG just now and asked about reducing the booking. They looked it up and said it's possible and to call back when I'm ready; but I'm inclined to believe you instead. I had an IHG booking in Japan where I had to arrive a day late due to changing flights a few weeks out. They couldn't change my reservation, but told me to call back after the stay to get my points back for the first night. Currently waiting on that to successfully process, so maybe that's another option.