I've been thinking in changing my v-brakes to disc brakes. Now that my rear v-brake can't brake anymore I think that it is time change it. I've already seen this article. My frame is prepared for disc and, as I don't want to change my brake lever, I'll go with mechanical brakes (does this look ok?);
The only problem is the rear wheel hub that has not the six holes for the disk. Is it ok if I just change the rear wheel hub and keep the front one?
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.
Good luck!
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!)