I'm replacing the hot/cold knob on shower. The front plate is removed, but I can't get the middle part to disconnect from the hot/cold pipes.

What kind of water pipes do you have? If it is copper it could be that you valve is soldered to the copper pipes. It will take a torch to undo them. If it is not copper they probably have half unions at the bottom of the valve that will unscrew. How does your new valve connect to the water pipes?