No real cross-training required to go from PA28 to C172 except you don't climb onto the wing to get in. Take you a couple of hours at the most to get the feel of it. Don't let that stop you. Just because you trained on a PA28 doesn't mean you have to fly only that for the rest of your life. Consider this as a chance to spread your wings a bit if the opportunity presents itself.
Cheers,
Pitts2112