Healthy nutrition is a very important part of your dog's life. Switching the food your dog is eating is a part of this. Should you need to switch the food you are feeding your dog, it must be done carefully to keep your dog healthy. Dogs in general, have very sensitive stomachs and digestive systems. This is especially true with puppies and small dogs.
When you bring a new dog home it is recommended that you do not switch their food for at least a few days. They are already undergoing enough stress with the move and getting used to their new surroundings keeping them on the same diet will help alleviate another source of stress. You should always make sure you get enough food with your new dog or puppy to last at least 3 days, longer would be better. If your new dog or puppy was already eating a high quality dog food then it may not be necessary to switch what they are being fed.
If you determine that you need to switch the food your dog is currently eating it should be done so in a careful matter. Switching foods should be done gradually as to not upset the dog's stomach or digestive system. You should start by mixing a little of the new food in with the dog's current food at each meal. Gradually increase the amount of the new food you mix in with each feeding, while reducing the amount of the old food. After a couple of days the mixture should contain almost all new food and barely any of the dog's old food. Finally, you can eliminate the old food entirely and feed your dog only the new food.
By gradually making the switch, you should minimize the likelihood of an upset stomach or other ill effects from the new food. Remember this process needs to be followed any time you need to change your dog's food. It should also be noted that changing your dog's diet should be done minimally, only when absolutely necessary.