I agree, budweiser. Why should conventional military units upgrade to what are essentially ragtag unconventional forces? Since we now have TOW infantry, perhaps these could upgrade to Balou's Modern Infantry or a similar unit, with a somewhat weaker attack than TOW infantry, and much weaker on defense. May need another unit in the upgrade path, as modern infantry will come rather late.
This would need some coding, but I would like to see perhaps 1 or 2 guerilla units generated when a city is conquered, ala civ2, though not in the numbers Partisans were seen, that was really too much, grabbing a city and having 8 or 10 of the pillaging little vermin pop up. Also, when you change governments, the revolution could spawn a few barbarian guerillas from your own cities (if your culture is strong or you change to Democracy they could flip to you after the revolution). I posted an idea like this in another thread about barbarians, where if you have a city go into disorder it will spawn barbarian units that may attack you or nearby civs.