We tried both paths. Here is what we learned, what changed in the pack's energy and what really matters when choosing.
What works for us
Every dog is unique: age, activity, weight and sensitivities matter a lot. We always change food gradually, over several days, and we watch the coat, the energy and the poop (yes, that too).
When in doubt, talk to your vet before changing your dog's food. And remember: snacks count towards the day's calories too.