AFAIK, you can only purchase the dining plan as part of a vacation-package purchase from Disney. But I sugget that you call WDW Customer Services to ask if your tickets can be upgraded -- (407) 939-6244.
From
www.mousesavers.com ...
The price of the regular Dining Plan has been reduced by $1 per person per night, meaning the 2008 plan costs $37.99/adult and $9.99/child per night. However, it no longer includes gratuities or appetizers (but tax is still included). This is effectively a 15%-20% decrease in value. However, the Dining Plan remains a good deal for many people.
A new Deluxe Dining Plan is available for $69.99/adult and $19.99/child per night. With this new plan, each guest receives 3 meals and 2 snacks per night . You can choose from table service or counter service for each meal (so if you can eat that much food, you might as well eat all table service meals). The table service meals include appetizer, entrée, dessert and non-alcoholic beverage. "Signature Restaurants" still count as 2 table service meals with this plan.You also receive a refillable mug per person, to be used at your resort.
A "Wine & Dine" Plan can be added on to any package that includes dining for $39.99 per night. This allows you to enjoy one bottle of wine per night from a selected list. Click here to see the brochure for this option.
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For most people, the Magic Your Way Package Plus Dining is the best value. To add the dining plan to a basic package in 2008, you pay only $37.99 per night per adult, $9.99 per night per child (ages 3-9). For that amount you'll get a full service meal, a counter service meal and a snack each day! That is a terrific value -- it is easy to get your money's worth.
The best part is that the dining plan is flexible, allowing you to swap a sit-down meal for a Character meal, or use two sit-down meals to attend a dinner show or eat at one of the higher-priced restaurants. You also have flexibility in when you use the meals. If you want to eat three of your sit-down meals in one day, you can.
The higher level packages are really luxurious, but for the average family they are also serious overkill. First of all, eating the included three full meals a day is just too much for most people. But if you're like me, you'll feel compelled to eat all those darn meals, because you're paying for them!