I side with your vet, I think it's best to keep them on a food they are doing well on. The yorkie is known for having a week intestinal system, and food changes are hard for them. I think the idea of a rotation diet comes from human nutritionists beliefs that say we need to have a wide variety of foods, but this idea came about because early nutritionists believed that was the best way to insure we got all the vitamins we needed. There is no evidence to suggest that a wider variety of diet is healthier if the diet you are eating is complete. For example, one of the healthier human diets consists of seaweed, fish, rice and turnips, that all they eat their entire lives, and they are said to be the healthiest humans, they live well into their 100's and are fairly disease free. Of course, American companies are looking at the turnips as the secret for the health, but I would say it's their diet and lack of bad things in it. The main thing is we really can't compare humans to dogs, humans don't have diarrhea when a new food is added to their diet, and most dogs do. I like this site about dog nutrition:
The Dog Food Project - Myths about Dog Nutrition