Those vet allergy foods bug me. Most cats and dogs are allergic to corn or wheat or soy and those foods are usually loaded with at least one of them.
The best way is to take your old food's ingredients and find a food that goes the opposite way. So if you have chicken and corn and wheat, you get a food that is salmon and rice or lamb and rice. You know what I mean? The ingredients of the vet food are so bad that it's a mystery why any animals at all ever eat it.