Cathy,
I agree with suggestions to feed a variety of food, ingredients across a lifetime. I also believe fresh foods, homemade feeding with high quality ingredients you can select/control can provide the best diet.
The idea behind these two things is the same as striving for optimal health in a human by eating a variety of healthful foods with various health benefits — nutrients, antioxidants and anti-cancer, immune system boosting foods, etc.
The truth is we really don’t know enough about all the things that produce more long lived, optimally healthy dogs. We don’t know nearly enough — and the research is far more limited in dogs & cats than for humans — about all the things that contribute to increased cancer risks or that prevent cancer, despite the very high percentage of dogs & cats that get cancer and that die from cancer. Personally, I incorporate what I know from human information on diet for cancer preventative, immune system boosting, anti-viral, etc. foods and environmental toxins and from past experience with a veterinary cancer specialist.
As Haley noted, dogs can and do commonly develop allergies to repeat insult/exposure of foods. There is veterinary research to support that.
With my food allergy dog, I currently cautiously add hypoallergenic health-promoting wholesome fresh fruits, veggies, quality oils to my dog’s commercial LID (novel protein) dry + canned diet. I needed to get her stabilized and healthy first.
But I eventually plan to move toward feeding, under veterinary specialist supervision & direction, a rotating mix of balanced homemade meals. If that kind of thing interests you, the vet nutrition specialist (board certified) I would recommend is Susan Wynn; she can consult with your vet long distance.
As far as how quickly to switch, that really depends on the individual dog and breed (some are touchier digestively than others), whether you are switching to a much richer (much higher protein and/or fat) food or one with very different or special ingredients that might cause upset, etc.
