Hi Thomas,
If you want to upgrade your FREE Dining Plan to the Deluxe Dining Plan, you can absolutely do this and simply pay the difference between the cost of the Deluxe Dining Plan and the regular Dining Plan even though you have received it for FREE. My family have recently 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 some of the popular restaurants may be booked up already.
Dining reservations can be made online at the Walt Disney World website
HERE.
Wishing you a magical trip filled with memories which will last a lifetime!