Can changing my dog’s diet suddenly cause harm? - Countryside Veterinary Clinic

Yes, we always recommend that you take at least a week to transition your dog's food from one food to another food. It gives the bacteria a chance to change which bacteria it needs to break it down. Sometimes the fiber content of one food is different from another. We don't want any shock to their little gastrointestinal system. So make your changes slowly and I think you'll be more successful.

Can changing my dog’s diet suddenly cause harm? - Advanced Animal Care

You’d mostly see a lot of diarrhea, so we recommend making that food transition over at least seven days. Because if you think about it, your dog's GI microflora, those happy bacteria that live in their GI tract, generally sees the same thing day in and day out. And when we make a sudden change, those guys are like, whoa, what is going on here? And it can cause some changes in the consistency of their stool and possibly some vomiting. Dogs’ systems don't like that change, so we want to make that transition slow.

Can changing my dog’s diet suddenly cause harm? - Animal Hospital of Statesville

It can. Usually, you'll see intestinal upset because the microbes in their intestines don't like quick change. They need time to adapt to new food substrates and levels of carbohydrates. Gradual mixing of foods over 7 to 14 days is recommended. There are a few instances where we'll recommend going straight to other food because of health reasons.