What about touching base with their other FM/OB docs and potentially filling in locum shifts for them before you start? Usually, you need proof or a certain number of cases proctored. If you ask them for a few weeks and help getting your feet wet there ie by being proctored, or being primary with another attending being second assist that might help with some of the nerves. I would bet they'd rather help you get comfortable and transition rather than have to recruit another doc.
It sounds like you're scared of the unknown and getting some experience there can alleviate that.
Now if you don't *want* to do it that's a different story entirely.