In your situation it is possible but requires the hotel to be willing to work with reservations to do it. The hotel will have to manipulate their inventory to be able to make the change. I've had success in doing it previously but it's totally up to the hotel staff and it may need to be a manager who does it as the access required is usually not held but the front desk staff who typically answer the phones. I would go ahead and book the standard room that is available, and talk to the hotel and say you recently got a cert and booked the night, could they please work with reservations to modify the other reservation to be nights 2-7. You have months to get the right person to help based on your timeline and that gets the FNC night locked in.