Your Fps and ADRs can be changed at any time up to and including the day that they're booked for. All that your Magic Band does is store the information saved on your MDE and to make it available at FP points or to show which Dining Plan you have (and your remaining credits).
You can actually even change your FP at the time it comes due -- just log on and select something else if you change your mind or change the time if you can't make it...