Switching to a new food can be a big change for an older dog who is used to their “regular” food. Ideally, don’t switch their food all at once. Instead, transition to the new food over seven to 10 days, or even longer if your dog is experiencing any digestive upset. Start on day one by replacing about 10% of their normal food with the new food. Every day after that, add another 10% of new food. If your dog doesn’t balk at the change, on day seven it is probably safe to switch them to 100% of the new food. If your dog experiences vomiting and diarrhea at any point during the transition, stop the new food and contact your vet.