How risky is it to drill your rims out (diy)?

As stupid as my question is, it was really an experiment. Here goes:

I have a trek 1000 that I don’t ride very much anymore (now I ride a few hybrids )and it has presta valves. I can’t stand them because I’ve had several snap off. I took my Alex rim (front wheel) and grabbed a metal drillbit and enlarged the hole in the rim so it will accept a shrader valve. Now however, I’m worried about trying it out because I’m sure that it can’t be that easy— I just envision that Schrader valve rubbing on a rough piece of metal around the hole and blowing out.

It’s just one rim and I went into it accepting that I may destroy the rim. Has anybody either tried this with success, or met with spectacular failure?