When I switch Levi's food, I do it gradually over about the course of about a week. The first three days, I feed 75% old food, 25% new. Then 50% of each for about three days. Then 25% old food, 75% new for a few days. And then he eats only the new food. This works pretty well for him. He usually will pick the new pieces out and eat them first.
He is currently eating Orijen, which I think is an awesome kibble. Orijen has puppy formulas as well.