Somehow I conflated promos and upgrades, my bad. Maybe heal to full in some situational scenarios could work, like if in a city? I think op is somewhat correct re the realism attached to upgrades, but I don't find it that much more believable that a unit with a couple dozen nearly beaten horse riders remaining in it somehow become a full strength tank squad, for the same price as a full health unit might. I kinda like the idea of only upgrading if full HP to work around this concern, and stop mid-battlefield upgrades, though this is somewhat off topic