I guess making a gradual transition would probably be easiest. One thing - you don't need to replace all the milk with milk (if that makes sense), they need food and milk is just a part of the diet. I've known peopel who have switched all the formula bottels for bottles of cow's milk and their kids just didn't eat as they got so full of milk. The milk isn't even essential, really, as there are other ways of getting calcium etc (if your child is like mine and refuses any milk but breastmilk )