I had a search across these forums about this but haven't found a topic so far;
I'm sure I'm not the only one who sometimes doesn't like a certain flag after you just got this awesome constitutional monarchy for your nation. You can keep your old flag after a revolution by editing the save game.
You need a ESF editor to do this, download is located here: http://www.twcenter.net/forums/showthread.php?t=242353
1) Open your save game with your ESF editor and navigate to: Campaign_env/Campaign_model/World/Faction_array/
2) Now you need to do some searching. Click on every "FACTION_ARRAY/FACTION" untill you find your nation and expand this directory.
3) Click on FACTION_FLAG_AND_COLOURS. In the first line you can change the flag set.
For instance for the United States with a constitutional monarchy you can change "data\ui\flags\us_monarchy" to "data\ui\flags\united_states".
4) Now save your file and enjoy the benefits of your new government without the loss of your beautiful patriotic flag!
PS. Since I'm kind of new here, I'm not sure I posted this in the right forum. (Thanks for moving!)
Edit: I forgot to mention that after having applied the above changes you will still see the "old" flags in the load save game screen in Empire(the two with your nation's leader in the middle). However once you've loaded your campaign all flags, both campaign and battle, will be showing correctly!