Quoting cardinaldirection,
I think anything should be upgradable via a in-depth system
I disagree. Speaking just for myself here...
I tend not to produce units that do not have a clear upgrade path, i.e. I give my line infantry strength and muscle even before weight is an issue, because I expect them to be around for leather and chain. When I build dodgers, I am careful about leaving enough encumbrance to upgrade to the first shield.
When I switch winged spears for lightning pikes, I feel that this is the same unit. I give them names like "1st Elusive Spearmen" and "7th Warg Archers", and I can track their progress across the game.
If I could upgrade a quick, lithe and balanced warg archer into a strong, muscled and defensive lancer, it would be completely nonsensical, and I would not feel as if I have improved my veterans. I will feel as if I have spent an amount of "experience resource" in order to change my army composition.
So yes, I would love an interface where you can change equipment of the same type any way you want, but I would not particularly care to have the ability to change traits or weapon types. I.e. short sword and tower shield to long sword and buckler would be OK (Although in real life it really isn't)