Which Hotel and how are you trying to change your reservation? I'm shortening my reservation at the Hilton by one day and all I need to do is make the change before I check in to avoid a $50 fee.
Watch what you do, I made a change and they decided not to give me the botcon rate and my room went from 129 to 179+taxs and such so now its 209 a night.
I had called the Hilton asking if I could change my checkout date to Sunday from Monday. They said if I waited till I arrived there would be a $50 fee for early check out. BTW when I went onto their site to change my reservation electronically, the site didn't appear to retain the convention rate. I plan to call the Hilton tonight and see what the deal is.