Experts have recommended that it is best to transition slowly to the new pet food. When you first begin you should have the pet's food contain 25% of the new food and 75% of the old food. After a few days, without any stomach problems, you can start giving your pet 50% new and 50% old food ratios. Then for a few days use 75% new and 25% old food. After that you can fully transition over to the new food. This total process will take around 2 weeks.