Well what might help is that the more advanced the unit the less armour upgrades it can get. Even with an Armour factor many of the Knights units are limited to one bronze upgrade so they would not be getting +6 or +9 (or something like that)...

What ever change it is need to be simple enough to be modded and even then I am not sure if it is possible as the upgrade adding +1 may well be hardcoded...

This would then require CA to change it which is incredible unlikely as it is not a bug (due to the fact that it is working as designed), it would be a feature change...