I did the same thing with Baron ! I bought the adult food when he was 9 months old. I gave it to him and he was fine. He's on Royal Canin # 27 now. It has a Jack Russell on it. So did the puppy food. The pieces were bigger, But he's grown too . It was fine. Check the ingredients, They should be the same or darn close. We had no upsets of any kind with the adult size... Hope this helps...