Thanks. Once I'm 100% certain I want to try to change the rez, I'll call the hotel manager and see what happens.
I'm not terribly optimistic because the "paperwork" might get in the way of common sense. Last year, I was in a Residence Inn by Marriott and "corporate" had assigned me a certain room type for my priceline stay. When I got there, I preferred a different style room, which actually cost less if you book direct (basically, I wanted a downgrade). The manager on duty couldn't figure out a way to put me in the less expensive room, even though he wanted to help.
So I can only imagine the difficulty of getting "the computer" to change the date!