You wanted opinions, here is mine
.
Upgrading should be banned totally, for the player and for the AI.
How do you convert a bow to a rifle, anyway?
I can see a small retribution in hammers or gold (probably gold) for disbanded units being brought in after currency, to allow you to get rid of old units.
Sure, you'll mourn those veterans, but I just don't see how a guy who learnt to destroy city defenders with his mace suddenly knows better to thorw grenades at city defenders, while other grenade launchers never can learn this.

Upgrading should be banned totally, for the player and for the AI.
How do you convert a bow to a rifle, anyway?
I can see a small retribution in hammers or gold (probably gold) for disbanded units being brought in after currency, to allow you to get rid of old units.
Sure, you'll mourn those veterans, but I just don't see how a guy who learnt to destroy city defenders with his mace suddenly knows better to thorw grenades at city defenders, while other grenade launchers never can learn this.