I disagree with you to stay within the Zignature line. If you compare ingredients of Zignature duck to Acana duck for example you will see they differ slightly so it's worth switching to see if it works better and it's probably not the healthiest thing for a dog to get the exact same ingredient dog food with only the protein changing for the rest of his life:
Acana Duck and Pear :
Deboned duck, duck meal, whole lentils, whole peas, duck liver, duck fat, lentil fiber, pears, pollock oil, pea starch, natural duck flavor, duck cartilage, pumpkin, salt,mixed tocopherols (preservative), zinc proteinate, dried kelp, calcium pantothenate, vitamin A acetate, freeze dried duck liver, copper proteinate, chicory root, turmeric, dried Lactobacillus acidophilus fermentation product, dried Bifidobacterium animalis fermentation product, dried Lactobacillus casei fermentation product
Zignature duck:
Duck, Duck Meal, Chickpeas, Pea Flour, Peas, Sunflower Oil (Preserved with Mixed Tocopherols and Citric Acid), Flaxseed, Dehydrated Alfalfa Meal, Natural Flavors, Potassium Chloride, Salt, Choline Chloride, Minerals (Zinc Proteinate, Iron Proteinate, Copper Proteinate, Manganese Proteinate, Cobalt Proteinate), Vitamins (Vitamin A Acetate, Vitamin D3 Supplement, Vitamin E Supplement, Niacin, D-Calcium Pantothenate, Thiamine Mononitrate, Pyridoxine Hydrochloride, Riboflavin Supplement, Folic Acid, Biotin, Vitamin B12 Supplement), Blueberries, Carrots, Cranberries, Minerals (Zinc Sulfate, Ferrous Sulfate, Zinc Oxide, Calcium Carbonate, Copper Sulfate, Sodium Selenite, Manganous Oxide, Calcium Iodate, Cobalt Carbonate).