Do not cold-swap. The bacteria in a dog's gut adapt to what they eat, and switching abruptly is the express route to a vet bill and a ruined carpet. Two weeks is the right window for most dogs.
Days 1–3: 25% DogPax, 75% old food.
Days 4–7: 50/50.
Days 8–11: 75% DogPax, 25% old food.
Days 12–14: 100% DogPax.
Watch for the stool first; it should be firmer by day four. Coat changes take six to eight weeks. Energy and the can-be-bothered-with-dinner factor usually shifts in the first ten days.
If you are switching a senior dog or one with a known sensitive stomach, double the window to four weeks and ease in slower. Worth it.