Related Articles. Transitioning puppies to adult dog food-small & medium dogs Learn when to transition your small or medium breed puppy to adult dog food. Adult Transition - 21 months This comprehensive guide teaches you when to make the switch from puppy to dog food and what to look for when you do.ransitioning puppies to adult dog food-large dogs. At times you probably wondered whether it would ever happen! But despite those occasional moments of mischief and misbehaviour, your puppy is now all grown up.