It is important to leave her on the food the breeder has her on. I assume the breeder has chosen a high quality puppy food. If after you have her you and your vet determine that the food is not meeting her nutritional needs then you can decide what's lacking. There are ways to add foods to supplement, too. Changing a yorkie's food is no small things they are very sensitive. I used the ebook from
Yorkshire Terrier Information Center | All About Yorkies. I have seen others recommend Yorkies for Dummies.
Enjoy your preparations!