S.L.
I don't really know. Is there another doctor at the same office you'd like to try switching to? That may make things very simple. But I think the best thing for you to do is to call the office and ask. Maybe it's lying, but I usually pretend like I'm a potential patient and phrase it so it doesn't make anyone feel like I hate them. You could say, "Hi, I'm looking for an obstetrician, and I've heard about OB contracts. Do you require one of those? And what does that mean? If I'm not happy with my doctor, can I switch to another doctor in the office? Can I switch to a different office, or would I be stuck?" You can get your questions answered without earning yourself a reputation of being dissatisfied and picky (if they would even remember that you complained).
But even if they do know who you are, they're probably flexible. They know that sometimes people just don't "click." With my first baby, we got whichever doctor in the office had an appointment opening. They said they hoped we'd meet all the doctors before delivery, because whoever was at the hospital that day would deliver us. I didn't LOVE that arrangement, but it was fine, until I met one doctor that I HATED. I have fibromyalgia, and when I asked him about it, he immediately treated me like it was all in my head and I was a mental case. So I asked not to see him again and they accommodated me. I was terrified he'd be the doctor on call the day I delivered, but I lucked out.
Good luck!