I have to take care when I change Coco's food, his stomach can't cope with the change over. When I bought him from his owners they had two adult dogs, the two pups had been given adult mixer and pal. He was four months old when he came to me and he couldn't pass motion very well at all, so I had to change him to puppy food which upset his system immediately. Then the change over to adult food.
I would only feed small amounts until his stomach settles.