Hi!I applied stanford for EE phD program originally. But they gave me MS ad + phD waitlist (They mentioned in the e-mail that I can accept waitlist by April 15th.) on Feb. 27th.
I have talked to several phD students in Stanford to know the chance of getting a phD ad from waitlist. And they told me that this rarely happens. First, only those admitted to MIT or Berkerley may want to decline Stanford. Second, they do not rank students on waitlist in Stanford. That is to say, even if somebody else decline Stanford, who knows who will get AD from waitlist?
However, if you really want it, it is possible to get phD finally after ms in Stanford. You need to pass the qualification examination, and to find a professor, that's enought. One of the students I consulted has just transfered from ms to phD with RA.
Hope this will be helpful for you.