You can absolutely upgrade the FREE Quick-Service Dining Plan to the regular Disney Dining Plan and simply pay the difference but this must be done prior to your arrival at Walt Disney World.
My family have recently returned from an 8-night stay at Disney's Port Orleans Resort and did the exact same thing.
If you decide that this is something which you would like to do at a later date, you will need to contact the company with whom you make your original holiday package reservation with. They will be able to tell you what the exact cost of upgrading your Dining Plan will be. (This will not change unless you are making a reservation for travel dates in 2013 when the cost of the Disney Dining Plans may increase)
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 at your earliest possible opportunity. 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 book up very quickly.
Dining reservations can be made online at the Walt Disney World website
HERE.
Wishing you a magical holiday filled with memories which will last a lifetime!