I strongly suggest continuing what your puppy has already been eating, at least for a while so their stomach won't be shocked. I slowly changed my puppy to Nulo puppy brand after the first few weeks, but she didn't have any stomach problems and she never had runny stool. Mochi stayed on Nulo until she was about 5 months old when I changed her to blue buffalo wilderness. After the blue buffalo scandal, I changed her to Origen and now that she's off puppy food she's on Costco brand food. She doesn't like it as much, but she'll get used to it and she's still healthy and having healthy poops. Nutrition is so important especially for puppies. I've read that if your puppy has runny poop, then they're not absorbing the nutrients properly.