But if you were hired at a certain commission, don't they have to keep you at that pay scale. No. The terms of your employment are subject to change. I doubt you'd be upset if they changed the pay structure at which you were hired to increase your commission rate.
So from what you are saying they can take as much as they want without telling us? They did tell you.
I'm not unsympathetic - nobody likes a pay cut. Your employer hasn't violated any laws however. You have the right to seek new employment elsewhere if the terms of your employment are no longer satisfactory.