Read your pill packet info-- usually it tells you how to effectively switch pills. Also, the Yaz website will probably have that information.

With most pills, however, an effective switch can be made as long as there are no breaks between switching pills. My pills said to simply start my new pills when I finished my old ones, and I would be fully protected.
I have mostly heard that you should wait a week, though. Your body is already used to the hormones, but it might need time to adjust to a new regimen. Wait a week to be safe, or even a month if it makes you feel better. But you cannot receive more accurate information than that of your doctor or the medical pamphlet of your pill.