What IRC said is important. You don't only need a new fork(yes,you can get a disk coimpatible one and get the brakes in the future), but you need Disc specific hubs, which might mean new wheels altogether unless you can build wheels and lace the hubs or you have a shop near you. Its just that now your up to even more money. Doing it gradually like you mentioned would probably be a good idea. Good luck! It can be expensive but worth it.