The valve under the large hex cap closes off the refrigerant at the condenser however when closed if you remove the schrader valve stem the coil and line set side is still open to the Schrader port and gas will be lost should you attempt to remove Schrader valve.

There is a tool however that will let you remove and replace your valve core without losing any refrigerant, it is called istrangely enough a valve core removal tool.

You can find one at your local HVAC supply house.