This has been done many times. I've ridden, but never owned, a disc brake bike, so I tend to be a bit biased. While there are clear advantage to disc brakes, I'm of the opinion that these advantages are overstated. I've never had trouble stopping with rim brakes. Rain or winter, I can induce a skid, which means I've exceeded maximum braking power. (In fact, most of the extra grip offered by disc brakes is counteracted by the extra force they need to stop the wheel...what rim brakes lack in gripping power, they make up for in better leverage.)

I'm not sure how far you have to ride for rim wear to be an issue, but I apparently replace my wheels for other reasons before I hit it (if the spokes are corroded to the point where a complete rebuild is necessary, I take that as my cue to just replace the wheel).

Other than that, I have a general dislike of the look of discs, and the extra-beefy forks that are needed to deal with the forces involved. Plus, I'd need a new bike rack for my car, since my current setup can't fit a front disc brake.

The main advantage of discs that seems to be substantial enough to notice in real life is the easy setup, adjustment, and low pad wear. I will admit that fiddling with toe-in to keep my cantilevers from squeaking is a bit annoying. If that's a big deal to you, maybe discs are worth it.