I did this today for a bike I'm sorting for a nephew. The MTB width (26") rims were drilled for and fitted with presta (6mm holes). He wanted schrader (and narrower tyres so I couldn't reuse the inners anyway), and a schrader valve (well the Michelin one I bought with the rubber sleeved bottom 2/3rds) needs a 8.5mm dia hole. So I used a 8mm bit (not 7mm) and wiggled it about a bit to get the last half mm and also to eliminate burrs on the hole sides. As @Racing roadkill says, carefully clean out a vestiges of the swarf and gently file the hole edges to make the job a good'un.
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