Aftyer 6 years I'd be very surprised if you have any recourse through the courts, I'm sorry to say. I wouldn't try to change it yourself, stonemasonry is very skilled labour and you'll end up making it worse, and in the long run you'll end up forking out for a new stone.
Also, getting the stone reground to that depth could weaken it and shorten the lifespan of the headstone dramatically (depending on how thick it is to start with)
I'm really sorry, but I can't think of a way around this.