Yes, the best way to transition to Young Again foods is to just replace his existing food with ours as long as he eats our food within 12 hours. His stool will be normal unless he overeats our food. If he does overeat our food, his stool will soften and have the consistency of mashed potatoes. This is not harmful and will correct as he learns to eat the proper amount of our nutrient-rich food. Most cats take only a few days to figure out how much of our food to eat, while others can take several weeks. He will metabolize our food just fine. The soft stool is because of overeating not how he is digesting our food.
I would cut his wet food down to a tablespoon twice a day once you are sure he is eating our food. We always say cats can’t count, as long as he has our food available at all times he will be content. The spoonful of wet just lets him know you still love him and continues a ritual he finds pleasing. He won’t need the larger amount of wet food because he will have our food available at all times and the wet will now just be a treat. Feeding our food free choice plus a can of wet food may cause weight gain.
Most cats eating our food will self-regulate and drop the excess fat. Sometimes they stay the same weight because they drop the fat and gain an equal amount of muscle.
The psychology of how cats perceive food in their environment has a lot to do with whether they overeat or not. Please read the entire back label of our bag, as it will explain the proper way to free feed our food.