If the future calls for a re-installation of Win7, create a bootable USB stick from your Win7 Enterprise DVD. (Google how to do that, if you need to.)
At that point you can only install Win7 Enterprise from the USB stick, *BUT* if you delete one file on the USB stick -- ei.cfg -- you will be prompted during installation which version of Windows 7 you want to install.
There is also the option to edit the ei.cfg file (again, Google is your friend), but for me, it was simpler to just delete it.
While you can choose any version of Windows during the installation, your Product Key is only valid for the type it was issued.