If I were you I would buy both and use a little bit to transition and then return the unused portion. Check the stores return policy first. When I worked in a pet store the food was guarenteed by the manufaturer. Even if someones dog ate the entire bag and then decided they didn't like the food they could return it for a refund. We just had to return the bag or unused portion. If you can't return it then I think you should buy both and give the unused portion to a shelter. I don't mean to sound rude, but the dog shouldn't suffer because you are cheap. Dogs can't vocalize an upset stomach like people can. Everytime a dogs food is drastically changed their tummy does get upset.