No, that's right...
I didn't get it that way, sorry.
But there is a solution (which might be limited by the amount of units available in the game btw)
As an example, in a mod I did for myself, I wanted the Genoese Crossbowman (unit ID 866 & 868) to be replaced by the legionary (unit ID 1) Not for myself, but for my oponent to produce and use it normally.
Selecting the Italian Civ in the Unit menu, I copy/pasted the legionary (1) on the Genoese Crossbowman (866 & 868). Then, when I played against the Italians, they produced Legionnaries instead of crossbowmen...
This technique could be used for what you're asking but it takes times. Using "unused" units (that are only used in the single player campains) : all the heroes, legionnary, heavy pikeman, heavy swordman, priest, ... you rename these units thanks to the technique i described above and you copy/paste them on champions, archer, knight, whatever you want for specific civs you want to alter...
Winter is coming.