i would also guess you don't have a cassette, but a 5 or 6 cog freewheel (with built-in coasting mechanism) that threads onto a rear hub. if that's the case, simply buy a 700c rear wheel or hub with the same standard threading (english, most likely) to receive the freewheel. if it were me, i'd go to a local shop and ask to borrow a 700c wheel to see how my current brakes reached the new wheel. it's helpful to do this both front and rear as you may find the smaller diameter fits one of your brake caliper's pads better than the other.