Since dogs typically eat the same food every day, their systems are more sensitive than our human ones. An abrupt change in food can be hard on some dogs’ gastro-intestinal systems. They may experience loose stools or diarrhea. The best way to avoid stomach upset is by transitioning, rather than abruptly changing, to the new food.
The gentlest transition is to make the change slowly over a few days. You can add 25% of the new food the first day (mixed with 75% of their previous food), adjusting the percentage over a number of days until they’re eating the new food exclusively. Sometimes this sort of controlled transition isn’t possible. In that case you can also feed smaller amounts more often for the first few days. Dogs are excited by new tastes and may gulp their food which can also cause gastro-intestinal symptoms.
It’s possible you’ll notice a change in your dog’s stool even after the initial few days. This is typically because of a change in the amount and sources of fiber and protein in the new diet.