<p>If I modify or add an environment variable I have to restart the command prompt.  Is there a command I could execute that would do this without restarting CMD?</p>

By design there isn't a
built in
mechanism for Windows to propagate an environment variable add/change/remove to an already running cmd.exe, either from another cmd.exe or from "My Computer -> Properties ->Advanced Settings -> Environment Variables".
If you modify or add a new environment variable outside of the scope of an existing open command prompt you either need to restart the command prompt, or, manually add using SET in the existing command prompt.
The
latest accepted answer
shows a partial work-around by manually refreshing
all
the environment variables in a script. The script handles the use case of changing environment variables globally in "My Computer...Environment Variables", but if an environment variable is changed in one cmd.exe the script will not propagate it to another running cmd.exe.