How to implement market spies (and tech downgrading)?

[jonbaer]

I am interested in a game where the markets (or resource counts) are visible to civs with specific "heroes", basically I want to create a hero which acts as a spy that can unlock seeing everyones resource counts (not just allies - like what gets unlocked in the markets).  My question is if this hero is killed can I downgrade a tech like that?  Is this something that is even possible?  I know there was talk previous on "spies" but I figured I would try to experiment w/ this route.

Edited March 17, 2017 by feneur
Tried to make the title clearer