I think it kind of depends on the dog. For example, my golden retriever is on Taste of the Wild, and I switch flavors (not brands) with no transition and have zero problems. With my German shepherd puppy, I never could have done that. He would have had diarrhea for a few days. If your dog is known to have issues with new foods, or has IBS, etc., it would be best to transition.
Forrest 9/1/2016 - 5/14/2017 RIP
UFO vom Wildhaus "Archer" 4/30/19
Brooklyn - Golden retriever 1/30/11