I have done this only once. And it worked then. For reasons that I don't understand, there were available rooms on higher floors with better views than the one I had been assigned, so I grabbed a different one. (I also wanted to check in on line because I wasn't arriving until the next morning and I wanted a room available when I arrived. Even so, one clerk said that they room wouldn't be available, but the clerk working with me said that I had booked for the preceding night so that it was.)