I switched my cats from Royal Canin Indoor 27 to RC Light 37 last week. Bickford is 23 lbs and the vet would like him to lose some weight. Bob isn't having a problem with the new food but Bickford is spitting up after eating from time to time and is less enthusiastic about going to the bowl. Should I switch back to the regular food or mix it with the new food, or do I need to call the vet?