Yes, the EN-200 ethernet modules can be installed in the field.  In 2010, an ethernet conversion kit was created to be used with the F3 version of the HP-2000 hand reader.  The only cases where ethernet is not compatible is with the F1 version HP-2000 hand reader and any HP-1000 hand reader.  It is not compatible for ethernet communications.

The F1 version handreaders have a blue or red box with 5 white dip switches in the corner of the mainboard; the EN-200UP-HP2K ethernet upgrade kit for the HP-2000 is not available for the F1 version.
The F3 version handreaders have a small, white reset button in the middle of the mainboard,
right where the colorful ribbon cable plugs into the mainboard.