If nothing else, I would continue the formula for nutrition's sake. Kids don't always eat their best at WDW. Heck, I don't always eat my best at Disney! Most of the kids' meals are devoid of much nutrition, and as parents, we tend to be more lenient on sweets while on vacation.
Powdered formula is easy to carry around the parks! There are containers which hold enough for three 8 oz. bottles. They are inexpensive. Simply fill the compartments before you leave the room each day! Bottled water is readily available in the parks, and we would ask for warm water at restaurants/counter service with no problem! Just have them mix hot water for tea and cold water or ice in a cup for you. Or better, get your baby used to lukewarm or cool bottles. There's no need to sterilize your bottles for a healthy, older infant. Only if you have a newborn or immunocompromised infant. If you use liner-bottles, the process is even easier! Just carry one holder, clean nipples, rings and liners in a baggie for fresh bottles wherever you go!