I think it's fair to say that JWST isn't just a tweaked KH like Hubble. The mirrors and sensors in JWST are completely optimized for infrared, which is a borderline-useless part of the spectrum for high-res terrestrial surveillance. Optical scopes also wouldn't need the extensive sunshield and cooling systems that JWST will. By the time that a theoretical secret optical JWST-KH were retrofitted with new mirror designs, new sensors, and new active and passive cooling systems for a public infrared science mission, you basically have a completely redesigned satellite.