[jakes mom]

You could probably go to a 5 star but just go very slowly. Take a week or 2 , mixing a little more of the good stuff in, decreasing the dog chow. Might get some GI issues but going slow will help that.
How old is the aggressive dog? Hypothyroidism is fairly common in older dogs and can cause some personality changes.
Definitely talk to the vet, the office may be able to refer you to a local trainer to get you started. Good luck!