Introduce the new food gradually. This is the most successful way to ease your cat into the diet change. Start by mixing 25% new food with 75% current food. Slowly change the proportions over the next three days or so by gradually increasing the amount of new food and decreasing the amount of current food. At the end of this weaning process, you should be feeding 100% of the new food. You may encounter difficulties; for example, your cat might choose to eat only the old food, or not eat at all. However, a healthy cat can miss meals for a day or two with no ill effects.