Part of the cost of Hubble was that the teams working on the keyhole satellites was not allowed to tell the Hubble designers about "lessons learned" and the Hubble team had to find them out the hard way.

Any links or references? Sounds like a fascinating backstory.

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.