If you have a package booked (room and tickets and/or dining), you can make any changes without penalty (and subject to availability) prior to your final balance becoming due which is now 30 days prior to check-in. Changes made within 30 days may incur a $50 penalty, but they typically waive it if the change costs the same or more than your original package or room.
If you book a room-only, you can change at any time prior to check-in without penalty. If you outright cancel within 30 days of a package or 5 days of a room-only, you lose your deposit, so it's always better to simply change dates than cancel outright if you are within penalty dates.
As the PP mentioned, changing dates may change the overall cost of your resort room-only or package.