Hi Maureen,
I understand your confusion. Because Cast Members occasionally allow guests to make substitutions when using their Disney Dining Plan, some folks have published that this custom is universal and can be expected at all quick service dining establishments at Walt Disney World. That, however, is not the case.
When using a credit from your
Disney Quick Service Dining Plan for breakfast, you are entitled to one entree and one non-alcoholic beverage. At lunch or dinnertime, you will receive one non-alcoholic beverage, one entree, and one dessert per credit. You may certainly ask when you order your meal or at the time of check-out (in a food court setting) if the Cast Member working there would permit you to make a substitution but typically they will not allow you to do so. It doesn't hurt to ask but be prepared for a very polite "no" in most situations.
The other option, however, is to use one of your snack credits to pay for that additional drink. This is a great way to enjoy that extra beverage without having to pay out-of-pocket.
Wishing you a truly magical vacation! Let us know if you need our help as you finish making your travel plans.
Jill