Smaller S/C Pulley

The Roush uses an Eaton M90, which utilizes a keyway to hold the pulley in place and not a press fit pulley like the larger M112. So yes, swapping the pulley is as easy as taking the tension off the belt, sliding it off the pulley, removing the nut, sliding the old one off and sliding the new one on.

You'll have to be conservative with pulley diameter though. Get too small and you'll require a complete retune. You might also have to worry about belt slip, since you're now increasing the slack in the serpentine system. Your tensioner may be able to make up the difference, but only if you remain conservative with your pulley selection. Another reason not to go overboard on the pulley size, is that you're now decreasing belt wrap around said pulley. With less surface area for belt contact, slipping the serpentine belt could be a problem.

...and when you do get it changed over, torque the supercharger nut to about 50 ft.-lbs of torque.