Used to be common on MBs--usually no problem
Rack him up, Dave--where's he get the guts to ask a question you think is dumb?
Reaming out the holes used to be common in the early days of mountain biking, because Presta tubes were hard to find. If your rim was drilled for them, and the only spares your friends were carrying (or that you could find in the sporting goods store) were schrader, you were screwed. As somebody else said, there's a reamer made just for that, or you could use a drill bit that JUST fits into a schrader hole (I forget the size, but Home Depot will have it). I've done several of them with no trouble.