It already is changed. I broke the rules. A lot. There are only supposed to be one, but I died and came back to life — because I drowned — which called the next Slayer. And she died, and then came Faith. And then years later, all of the potential next Slayers were being slaughtered by the most evil of forces, so I changed the rules with a lot of help from my very powerful witch friend and we made all those potential Slayers into Slayers.
So it's already different.
I get your point, but without that, I don't really have a purpose. I've been doing it for half my life.