Final payment for a package reservation is due 30 days before arrival. Prior to that day you can make any changes to the package without penalty. Once you are within 30 days of arrival you can still make changes, but you may incur a $50 change fee which is usually assessed on packages that now have a lower total cost, but if your total goes up the fee is typically waived. If you make any changes within 10 or 14 days (I forget which) of arrival there is an additional $15 paperwork fee that is charged regardless of whether the total package price goes up or down.
Changes to the dining plan can be made up to 3 days before arrival for a regular reservation and 2 days for a DVC points reservation.