It’s very easy! Once you start getting near the end of a bag (say a weeks worth of food left) you buy the next food you are wanting to try, and mix the new food with the old food, increasing the amount of the new food given as much as your dog will allow. Some dogs need slower transitions than others, just watch his stool, and as long is stays relatively firm, you’re good. Eventually, you will be able to switch the dog with little to no transition at the end of each bag. I’m fairly sure mine no longer any transitioning period, but I do it anyways, for a couple days at least.
