It is possible to change it online. You just open your reservation and there is an option to modify your reservation (and change your said date). I moved my dummy date online for my 2011 cruise, but it was several weeks after the booking period started. I'm not sure if you will have trouble doing this tomorrow with all of the people logged in pricing out cruises.
Thanks. I just try modifying my ressie online....yes, it kept 10% off ($70XX web price vs the $63XX). But it doesn't allow me to choose room and switch dining time. It kept my original settings (GTY, second seating) as being assigned by the onboard manager.
I guess I have to call....