I think the most realistic option is for UG1, UG2, etc. models of units to end up with more or less equal equipment. For instance, an un-upgraded medium infantry might have everyone with a spear and shield, but helmets and grieves sparsely peppered, and little to no torso protection. After a single upgrade, everyone has a helmet, half the people have a mail shirt. After a second upgrade, all of them have grieves and helmets, almost all of them have either mail, linothorax, or some kind of breastplate.
It makes the most sense to me, because when units are upgraded to use better equipment in the game via retraining in a settlement with a higher tier armory, they reasonably should have better equipment across the board on their models, just not fully standardized.
It'd also possibly help with the problem of having one guy kitted out head to toe in scale and mail standing next to a guy with barely a scrap of cloth for modesty in the same unit, which I don't think is very realistic or logical from any tactical perspective. Certainly, both might be on the field, but one would be put into a shock or front line unit and the other made a skirmisher or reserve. Not everyone could afford mail, but not everyone could afford a horse, they weren't sent to run while clapping coconut halves along side those that could.