You should switch dogs over to new foods gradually to avoid an upset stomach, which is uncomfortable for the dog and can be unpleasant for its human! On the first day, replace 25% of the old food with the new food. On the third day, you can feed 50% old food and 50% new food. On the fifth day, feed 25% old food and 75% new food. By the seventh day, it's safe to feed 100% new food.;