Changing food abruptly for a puppy often results in some GI upset and loose stools. Stress, like being yanked away from your mom and littermates and introduced to a completely new enviromment with a new set of humans, can also cause GI upset and loose stools. That's the reason for waiting 2 weeks before switching before introducing new types of food. It's best to let the puppy get settled for a couple weeks, then gradually introduce the new food, mixing it in with the current food in slowly increasing amounts. It just makes it easier on your puppy.
Lots of people never use puppy food, But anytime you switch food it's best, if possible, to introduce it slowly.
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain