This has happened to me plenty of times. This is what i have done.
You clearly found the rate acceptable hence why you booked it, the rate has gone lower. Thats annoying. agreed.
My suggestion ( This is what i do)
Give hilton a call. Say, I was looking at booking a better room and noticed that the rates have dropped. I thought my room booking was flexible. Is it just possible to change my room booking instead of changing my rate).
Better room, plus a potential diamond upgrade...