Hi Hilary,
You can absolutely upgrade your FREE Dining Plan to the Deluxe Dining Plan and simply pay the difference. My family have just returned from an 8-night stay at Disney's Port Orleans Resort and did something similar by upgrading from the FREE Quick Service Dining Plan to the regular Dining Plan.
If you decide that this is something which you would like to do, you
will need to contact the company with whom you made your original vacation package reservation with. They will be able to tell you what the
exact cost of upgrading your Dining Plan will be.
It is well worth the extra money in my opinion as there are so many
fabulous table-service restaurants at Walt Disney World to experience.
If you do upgrade, I would strongly recommend that you make advance
dining reservations for your table-service meals straight away. As
advance dining reservations can be made up to 180 days in advance, and
you are travelling during a time when FREE Dining is being offered, I
suspect that many of the popular restaurants will be booked up already.
Dining reservations can be made online at the Walt Disney World website
HERE.
Wishing you a magical trip!