Unfortunately, it isn't that easy. Graphic files aren't attached to particular units (like the Dragon Guards), but rather to a type of unit via model files. The model files determine how an element looks, a whole unit takes the visuals of the prevailing element type within it.
The blue cuirassier graphics are attached to the Gardes du Corps cavalry model, the red ones to the "ordinary" cuirassier model. If one were to correct the Dragon Guards to the red graphic set, the real "Blues" would appear red, too.
In order to implement historic looks for all the units off, one would have to create individual model files for each unit, as well as new unit files, implement those into all the scenarios and event files, ... I think you get the hang of it.