I've not done anything with them except the one you see (and some preliminary work on one other) but I have thought about how I am going to do it.
I am going to cut the rumps off, following the line between the barding and the saddle, moving around the chainmail. I think a good sharp no 11 and a razor saw will enable me to do that. Then, the fronts and backs can be swapped.
I will have nine models in my unit, so I will have five (three of one, two of the other) standard and four "swapped".
I think it should work fairly well - not as good as having more variations, but that will give me four variations, which is all you get with the existing Heavy Cav horses from GW.