If your cat is throwing up right after eating the new food, he/she may be used to the old food and respond better to that. I know my cat (she's 11 yrs old) eats dry regular cat chow however when I switched to Cat Chow for Indoor Cats w/ hairball treatment, she threw up right after. I thought it was a fluke until I picked up the pattern. So I switched back and she handles a lot better. I say if it ain't broke don't change it.