Old cat with food issues

On Sunday I am taking in a 12 year old cat whose owner can't keep because they are moving ( ) As mad as that makes me, I know the area she is moving to and there are not a lot of rental options out there. But anyways, the problem is that her cat, who is 12 years old, throws up on any food except for Meow Mix indoor formula. She told me she tried everything, by that I think she meant everything the grocery store sells for cheap. I know Meow Mix is crap and I bet it is an allergy.

I feed my cats Premium Edge. I am going to gradually switch her to that if she will eat it. As well as wet food, Wellness grain free kind.

Is there anything in Premium Edge that might spark an allergic reaction? An ingredient that is also in Meow Mix perhaps? I know this is the right thing to do, I gotta get her on a better food. I have also had good luck with Natural Balance for cats with food allergies.

Anyways, just looking for thoughts and suggestions... if any

Premium edge if meomory serves has quite a few veggies and grains any of which can trigger allergies...

Protein sources( not quality of ) they share is chn and fish ...

You may want to try moving meow mix to like purina one and then to the GOOD stuff ... older cats need REALLY slow adjust times.. Meow mix has lots of dyes which could easily be a allergen

Kandie ate junk food ... I ended up mixing Eagle pack and Nutro max to get the right flavors

All I have to add is that my Damita has food allergies & she is doing well on the Natural Balance Venison & Green Pea.