canti studs placed for 700c wheels will not work for braking 650b wheels, they are too high. the only exception is a very few modern bikes have removable posts, and as Will B said, there are bmx accessories that will allow a lower stud placement. these adapters will absolutely not work with traditional brazed on posts. i know Thorn in the uk has some forks with thread in canti posts, though I'm not 100% that they are compatible with the mentioned adaptors.
the canti posts are useful for mounting a mini-rack; otherwise most people either ignore them or grind them off, and use either center pull or side pull brakes with long enough reach for 650b.
just a thought: if the bike has canti brakes it's likely that it has wider clearances than a "racing" bike, and you might be better off just putting on the widest 700c tyre that will fit the frame.
cheers
Dave in NYC