Yorkies, being a small breed, need a nutrient dense food. They don't eat much so it needs to be a high quality and very digestible. I personally wouldn't go from RC to Purina One. All the varieties I looked at have corn gluten and by-products in it
. There are many better foods than that.
I use Nature's Logic. It's a very high quality food and all my dogs love it. It's not grain-free, but the only grain in it is millet. I like that because millet is not a starchy grain and I feel that some grains (rice, corn, oats) are not so good for the teeth. They provide carbs for the dog so break down into simple sugars. My opinion is that this can contribute to dental problems so I'm happy with the millet being in their food.
I feed all mine twice a day with just dry in the morning and dry mixed with a little canned and some water in the evening. Works for me and my pack.
Also, I give them chicken and sometimes liver for treats.