Okay I was thinking about a little historical context for the modernization of mesoamerican and southamericans, and i think the best way is establish some rules about how both cultures and religious were mixed, so:
1. No suits, because this was a way to venerate the animals who in his original culture was a religion thing and any priest who the faction have will be directly disolved.
2. They'll has some units that to get the new technology will needs an upgrade because his own level in the society doesn't allows them to get the new equipment.
3. All old religious buildings will get destroyed and they will be capable of build some western buildings like a church and maybe (if the game allow me) a wall.
4. The original top units like jaguars, eagles, arrows or cuahchiqueh will keep some details from their original model because their was from the high society so their privilegies will be respected and more of their will can conserve old heritage things (Look the images)
5. The posibility of change your religions and therefore the modernization will arrive at different times and circunstances, if Aztecs can maintain their empire untouched, then they will not have any reason to change their religion or modernizate (maybe after this patch I can see the posibility of a modernization without change your religion) but if they start to fall, that can be a option, with diplomatic consecuences of course, no one faction will have the same requirements.
So, just for example, aztec roster will be:
Peasants -> Can recruit but no modernization
Coyote -> Totally erased
Warriors -> Can recruit old ones but the upgrade will give them a much better equipment
Bodyguard -> Replaced by the second image since modernization happens
Spearmen -> Same that Warriors
Cuahchicueh -> Same that bodyguards
Spear Throwers -> Can recruit, but they will be a residual unit
Archers -> Can recruit, they will be a residual unit but will be a dismounted muskets unit
Eagles -> Totally erased and replaced by the unit of the last image
Jaguar -> Totally erased and will be replaced by a cavalry unit
Arrow -> Totally erased and replaced by the first image unit
Bonus: