actually, even if you switch brakes, you probably still won't be able to mount cross tires on your road bike -- there most likely won't be clearance at the seat tube and/or chain stays and fork for the larger cross tires.

but, depending on your courses, I've seen road bikes race cyclocross -- crazy if you ask me, but it can work if you don't have super-loose or rocky/rooty courses. here in northern california, some of our courses are pretty flat, hard-packed and fast (and typically dry until at least December).