How early would you be checking in on the new date? People often reserve and pay for a hotel room the night before if they're arriving early in the morning.

BTW I'm confused about the terms of your reservation. You say that it's refundable but talk as if it's prepaid. If it's just guaranteed with a credit card, you would normally pay Hilton at checkout so the Hilton promo should apply if the rate is eligible. Or do you have some sort of travel package? Also, what form of AmEx travel did you use? PTS/CTS, city travel agency, corporate travel office, etc.?