It IS possible to edit your COA (Coat Of Arms).
But you'll have to know its code, then make the change directly inside of your savegame (that you open with a text editor).
There are two ways of getting the 'code' of a specific CoA:
1. If you have the Ruler Designer DLC: Start a new 'test' game. Create the COA of your dreams with the Ruler Designer, then note the code associated with it in your 'test' savegame.
2. If you don't have the Ruler Designer DLC, you'll have to find a CoA ingame that suits you. Note the name of the dynasty, then search for the dynasty CoA in your savegame. Note the code associated with the CoA.
Once you have your code, search your savegame for your dynasty's Coat of Arms. Replace. Save.
Load your game. Play your savegame.
Voilà!