I've always been nervous about trying to change hotel reservations myself as I fear that there could be a glitch or some quirk regarding availability, so I tend to make a new reservation and cancel the old, which gives me good email proof of the changes, but at times gets confusing if I somehow manage to cancel the wrong one. If I can't make the changes I want this way, I tend to call my private line agent or concierge, or even call the hotel directly.