Thought this might be worth posting as I think it could work as a possible alternative to overhauling the upgrade system. Currently as each new tech is unlocked existing units can upgrade to the new tier and you are unable to recuit anything but the new "upgrade" units. Would it be possible to create duplicate (but none upgradable versions) of all the units and allow them to be unlocked with the tech that makes them obsolete?

For example you start with Cohors, you tech up and unlock an upgrade that turns them into Legio. Now with this proposed idea a "new unit" duplicate called "Cohors b" is also unlocked with that research, allowing you to now recruit both Cohors and Legio. For obvious reasons these duplicates will be stand alone, as in they can never upgrade. This seems like a decent middle ground between the new upgrade system and the lack of available units due to said system. This way garrisons continue to level, as can your core army if you want it too. It would in effect be a large unit pack which functioned in a way that allowed players to continue to access units they'd upgraded past.

How feasible it is to create duplicate units and have them unlock through the tech tree I don't know, just thought I'd share the idea as it seems like a good solution to the potential headache of overhauling Attila's entire unit system.