I understand that, but all the modern units (in Europe at least) have origins and traditions from the past. Example, in France, the fist Infantry regiment come from the Bandes de Picardie created in the 14th century. Well, they haven't anymore bows and swords or spears, but the regiment still keep the traditions. Same for tanks units called Cuirassiers or other Hussard. So, a unit could be transformed in an other modernized units and keep the Eagle of the fist one. Well, it works in reality. That is what creates the Esprit de Corps and push units to fight even when no hope of victory remains, or to protect the retreat of other units.
So, the only limitations to transfer the experience and the pride of one unit to another unit is technical. I don't know about that aspect. In the real life, you just to give the flag (or standard, or whatever symbol of the unit) to the new unit (sometime after a certain amount of time in Museum -The Invalides, in France). Probably more complex for a game.