Until now, if you wanrted the appearance of a model/unit to change over time [French in WW1 losing the Pantalon Rouge], you had to create a new model/unit and do script commands to upgrade.
Now, for new versions of the game engine [XXX.exe with creation date on or after October 17, 2015], you have a new method:
http://www.ageod.net/agewiki/SetModelImage
Enjoy!