What disc brake kit are you installing? Some kits require you to install a backing plate that replaces the old backing plate. That will require the axle to be removed. At any rate, you are probably going to have to remove the old brake backing plate. This requires removing the four bolts that secure the axle retainer to the axle housing. It is not a big deal however. You may run into a little of a problem removing the axle after you pull the four bolts. The axle can be pursuaded to come out by attaching a length of heavy chain to a lug bolt and with a whipping motion jerk the chain.
Vince