While I am not 100% positive, there is a very easy way to check. I'm assuming since you tried to take off the old rotor you already have a new one. Look at the center hole of the new rotor, if there is no lip for the nut to catch on, it can't be held on by the nut. I would be very surprized if the rotor was held on by a large single nut. Single nut wheel applications are generally reserved for quick change race situations.
Also, don't forget to change pads if you're swapping out rotors.
While I've never done pads on an IS the rear caliper plungers will compress one of 2 ways: 1) simple compression or 2) a turn and compress type used on many Audis, Nissans and Hondas.
Sorry I couldn't be of more assistance.
Brian