I think trying to set barriers is just overcomplicating things. You either go for upgradable units or you don't. An infantry unit is an army not one soldier. A ship unit is a fleet not one vessel. Over the course of one or more turns the equipment the solders, sailors or aviators get to use is replaced with more advanced technology.
It is not an issue of morphing a horse into a tank but a primarily horse equipped unit into a primarily mechanised unit over the course of years. Looking at the German army in WWII this should be obvious because they were very clearly using these combinations side by side, with a mechanised division having mechanised infantry, mechanised artillery, mechanised supply trucks, horse riding infantry, horse drawn artillery, horse drawn supply carts and foot soldiers all mixed into the one formation. Their goal of having a totally mobile force was never realised because they lost vehicles faster than they could produce replacements but the blitzkreig successes they achieved were partly because they were better equipped in this respect than their opposition.
It is not an issue of morphing a horse into a tank but a primarily horse equipped unit into a primarily mechanised unit over the course of years. Looking at the German army in WWII this should be obvious because they were very clearly using these combinations side by side, with a mechanised division having mechanised infantry, mechanised artillery, mechanised supply trucks, horse riding infantry, horse drawn artillery, horse drawn supply carts and foot soldiers all mixed into the one formation. Their goal of having a totally mobile force was never realised because they lost vehicles faster than they could produce replacements but the blitzkreig successes they achieved were partly because they were better equipped in this respect than their opposition.
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