I just went through this a bit and had to hunt this down in the Terms and Conditions. It probably explains the issue, as you need to be within 90 days of your original departure date, although they are not explaining it well:
For Bookings with the Fantastica, Aurea or Yacht Club Experience, one departure date change can be made
free of charge from the time the Booking penalty period begins, as stated in the above cancellation chart, until
30 days prior to departure. Any subsequent departure date change will result in a fee of $50 per person. The
new departure date must be within 90 days of the original departure date. Any change to a date that is more
than 90 days from the original departure date is considered a cancellation and is subject to the above
cancellation charges. Any change made less than 30 days prior to departure is considered a cancellation and
is subject to the above cancellation charges.
