I like the idea, but hate manually editing files to do something like that. I looked at doing it automatically like AoD does it. However that system is not only flawed, it also failed to work at all.
I just had an inspiration. What if the player's cached their unit art?
While it might be a bit faster (it's likely so minor that nobody will notice), the real bonus here is that during revolution we can write something else in the cache. That "something else" can then be new data in XML, which is not used by anything other than the cache generating code and the game will be able to switch graphics without file modifications.
I haven't checked the code to see how well this would work, but if I remember correctly, it would be within reach to do something like this. On the other I'm not 100% sure it will work.