To be frank it depends I personally never had a problem changing my animals food. But some animals have allergies or simply do not like certain foods. If there are severe reactions it could be bad food (remember all the pet food recalls) or simply a allergic reaction. The only way to know is to try. If a animal has eaten the same food for years they may snub or get ill from a new diet just like animals who never eat human food who then get some often have vomitiing or diarehia.