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Basically, that's what I had in mind, like in Heroes of Might and Magic IV, but so that the units are similar, and not, for example, spearmen and ballistae – two incompatible units. In principle, I would be happy with the system that was in "Tribes of the East" – two upgrades. But in Heroes of Might and Magic VI, there's only one. And what's the point of having an unupgraded unit then?