I can't say with CP, but I know with American, it makes your chances not as good if you are not traveling in Coach. They usually only overbook Coach, and they need coach volunteers. I HAVE been bumped, but only because 1) I was elite and so they made a special effort and 2) I said at the very outset, when offering to volunteer "I would like to volunteer, but I would hate to not be chosen simply because I'm in business class. Is there a way that you can keep a special eye out for me because I would really really like to stay if possible." They told me no problem, and then becuase of that when I was at the bottom of the list (because i was in First class) they made a special effort and moved someone else up to First and let me stay.