I wouldn't suggest decreasing the protein and replacing it with, or increasing the carbs at all. Carbs should be used judiciously, not abundantly. What you should do if you're looking to increase their fuel source, is Increase the fats via animal or oils or both. Fats are a much better fuel source for dogs. Veggies are a good source for vitamins and minerals, (depending on what you're feeding, and what for) but I wouldn't feed copious amounts of them either. Dogs thrive on proteins and fats, although carbs have a time and place if you know what you're doing. Carb loading however, never has a place or is needed.