Shelley I tend to err on the side of caution and when changing food I give no less than 2 weeks unless its something drastically needing change. I usually do it over 3-4 weeks with Met though. My mom tried the 1 week switch over with her new healthy at the time pups and it backfired badly because it was just too fast a move from one food to another. She was trying to go from Solid Gold to Evangers at the time. She gave up and got conned into a veterinary formula with my previous vet. Anyway I hope it goes well. I have heard great stuff about Merrick- they just don't have food Met can eat. Between his allergies and the things that trigger his epileptic condition I am pretty much stuck. I wish there was a kibble out there he could eat. I was pretty irate when Solid gold added natural flavoring to the food and nearly cost us for it. Its a trend all of the premium diets are taking and I was so mad. The only company that does not use it or rosemary and has a diet that is free of all the common allergens plus soy, yeast, and the items already mentioned has something in the herbal aspects of the formula that actually makes my gentle man turn very temperamental! One day I hope to have a simple dog LOL Anyway I hope it goes well getting rid of Science Diet. Karyn and Met