This idea may just be the fastest way to strip a bottom bracket shell. BSA thread are slightly smaller than 35mm, so you start with a sloppy fit. Then the pitch error between 25.4 tpi (1mm pitch) and 24tpi ensures that the tap will be shaving the sides of the thread profile, and the threads will be 180° degrees out of phase over the span of about 3/8 inches.
The only way to convert a French BB to BSC is to fill the threads with braze, then ream and tap fresh. Of course that means burning off all the paint for about 6" in every direction.
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