Originally Posted by liquefied
They probably meant wheel instead of rim. Newbies tend to mistake the two and assume a rim on a bike is the same as a rim on a car.
Exactly. To use disc brakes you's need a hub to which you can attach the rotor (the disc that the brakes grab on to in order to stop the bike). If the current wheelset does not have the attaching points for the rotors, then a new wheelset is necessary.
My question is, howeveer, why change from V-brakes to disc brakes?
Unless a rider does fast, downhill descents, where the disc's heat dissipating ability is superior, then there is no real good reason to change from V-brakes to disc brakes.
A good set of Shimano XTR V-brakes ought to work well for most situations AND they are lighter than most disc brake applications.