Pull off the entire assembly and press out the studs to remove the rotor from the hub.
As far as that goes, I doubt you have them (the rotors) worn down to the point they actually need replaced even if they are grooved, I'd just have them turned.
Even a new "quality" rotor can very easily be warped (seen this many times over the last couple of years working at the dealerships), a lot of the time they turn the new ones immediately to eliminate the wobble.