I switched when I was pregnant with DD, to my current OB. My husband was there during our nephew's birth (they asked him to tape it) and loved me, lol. I just called them, told them my situation (lost blood work with low progesterone.....), and they made me an appointment for the next week.
My other SIL also switched to my OB during her last pregnancy, and again, just called and made an appointment, and she was 20 some weeks when this occurred.