The focus and recompose technique - along with finding alternative focus targets - is something we learn to do without hesitation. I find the back button (AF-On) focus approach makes it much easier.
I found moving from 51 pt AF to 39 point AF was irritating. Even more, the center and adjoining AF points are the most accurate so focus and recompose is a way of life.
The FX cameras have a shallower apparent DOF than DX cameras, so you'll probably need to stop down more often to get enough of your scene in focus. It's typically about 1 1/2 stops. Or you can emphasize the shallow DOF and smooth backgrounds. But you need to be very conscious of focus planes for groups.
I assume you still have two cameras. Two cameras can be a way to balance the difference between FX and DX.
Thanks for sharing your experience. It's a good example of the magnitude of change in moving to a D600.