That's a tricky question, and when trying to decide for new unit stats, I've almost been despaired.
I can't get it right...
But before moving to my problem, I'll try to answer your question.
For the early game, it is easy.
Bronze weapon and armor are inferior to iron, medieval long sword are better than a gladius (longer range), and chain mail / full plate are clear upgrade.
So we can basically consider that units are indeed geeting stronger until the medieval era, except in some cases (where training could make a difference).
The renaissance era starts to get tricky. Are pikes with no armor stronger than medieval pikemen with armor?
I think they shouldn't. That's why my pikes for "around 1650" are in fact the same strength as the medieval. But they have a +1 HP to simulate larger units than in the medieval time.
For the cavalry, that is indeed a tricky question. Is a Napoleonic cuirassier formation stronger than a knight? Well, as they had also carbin or pistol, we could consider they are indeed stronger.
But you'll notice for the 1700 heavy cavalry, as firearms were not so widespread, I made them only sligthly stronger than knights. I would have made them weaker for realism, but for gameplay I wanted to keep the units "upgrading".
Now, my really tricky part. Something that annoys me a lot.
As a general rules, I have roughly added 50% to a unit when it upgrades.
So a unit with A/D 10/16 will upgrade to 15/24.
Initially, the spearmen / infantry lines had a better defense than attack.
But I ended with something like
WWI infantry: 10/16
WWII infantry : 15/24...
And so if WWI attacks WWII, chance of succes = 29%, but if WWII attacks WWI, then chance of succes = 48%
I found it strange... A WWI vs WWII should give similar result, there's no reason why would gould attack much better than defend.
So I've made basic infantry : Attaque = defense
For assault troops (grenadier, engineer) and for cavalry/tanks, I've given an attack bonus, and for archer/voltigeur/MG a defense bonus.
For marines, I have a slight attack advantage, and for paratrooper a slight defense advantage (so they can "hold" the position they capture). And that's also to preserve balance.