Conversion kit? Maybe it's because I only work part time at a shop but I've never heard of a concession kit. Your fram would have to have the mounts for it. Not to mention that even if such a product existed, it would be pricy with the cost of new parts (levers, calipers, rotors, wheels, plus labor). Almost would be worth it to sell the bike and just get a new one.
I still hate the idea of disc brakes on rotors.