Typically, the front brake is more important if not as important as the rear brake, so you'd be better off swapping to a front disc (plus, if you are buying genuine Avid BB7's, they're sold as a front and rear kit IIRC). If you don't have a disc ready wheelset, you just need to get a disc ready wheel set (the front wheel is relatively cheap compared to a rear wheel) - I don't really see the point of reusing a rim in this case, since it will likely not be cost effective [and I never approve of reusing spokes].
Also, check the condition of the levers and use new cables + cable housing when replacing the brake.
P.S. It is spelled "brake", not "break".
(Note that this will all be significant in cost relative to putting on a new V-brake (Avid Single Digit 5's are under 20 dollars a piece. If you put in a new FR-5 lever and new cable housing and cable, you'll still be under 40 dollars - this may be a better option if you were satisfied with v-brake performance [obviously, you can opt for better V-brakes, pads and levers for not much more, but the point still stands that a good V-brake is significantly cheaper than a disc conversion, and decent levers really aren't that expensive]. Make sure this replacement is economical on the bike you're using - in some cases, with cheaper disc-ready frames, you're actually better off selling the current bike and getting something with disc brakes on it already!)