Psssst, folks - DX bodies use FX lenses just fine!! You can keep the body and still upgrade and share glass. With rare exception all my glass is full frame, including the lenses that spends the most time on my D7100.
Before you go selling the D7100, spend a week shooting the D810 in DX mode and see if you can live with it. It's 2 fps short, and after years of shooting with the entire viewfinder it's amazingly difficult to adjust to one that's 44% the size and keep things framed. If you can then kudos. If not, then you have your answer.
