650B is 584mm diameter, 650C is 571mm, so 13mm, or about 1/2 inch.

I'm guessing that most brakes will adjust, assuming the current brake shoes are not at the bottom of the adjustment slot. but you do lose b-bracket height and pedal clearance. Note that Rivendell tends to make bikes with low b-bracket height to begin with.....

Both Sun and Velocity make 18-19mm width road rims in 28 spokes. $40 -$60 at Sheldons.

Michelin and Conti both make 650C x 23mm tires.

Definitely check a wheel to see how it fits. Maybe ride them a bit to see how the bike handles. It always amazes me how minute changes on the bike can make a huge difference, not always for the better.

Steve B.