You can train hussar to be a tank crew member. It was done between the World Wars. You can train (almost) every body from every thing.
The transfer of traditions isn't about the persons but the symbol. The Marines of today aren't the ones who fought in Guadalcanal or Iwo Jima but I supposed every Marines has these battles in mind, as a part of their fighting spirit. It is a kind of cult of the ancestors, I supposed.
To come back to the game, I also think it will be nice to keep the well trained and old units to fight others battle. It was how Julius Caesar won against Pompey. His Legions were better. And a bit of luck, according some historians.