The upgrade mechanic is flawed in many ways. The biggest way that bugs me is this. If upgrading a unit costs half the resources and manpower of building a new unit, Shouldn't those resources and manpower also go into healing that unit? If a unit at 50% strength upgrades, It should be healed 25% for every upgrade level due to the infusion of Men and Materials.
Then where does all of that manpower go?
A unit has 2 game qualities.
1. Combat and movement Statistics.
All of the fresh resources used to upgrade a unit is where the Materials come from to upgrade the equipment.
That accounts for the increased attack, defense, and movement. Infuse a bunch of .30 caliber machineguns into an Infantry unit and its attack and defense value goes up. That requires 0 manpower.
If a unit is created with a 1000 manpower and it is at 50% strength, then the logical assumption would be that if you added 500 manpower to the unit, it would give that unit healing do to the infusion of live bodies where there were dead bodies before.