Originally Posted by
SquidPuppet
That's a 38mm reduction from 700c. Handling and steering will change. I suspect a pretty noticeable amount. You might check on Bike Cad or with a frame builder to get the exact number of how much trail reduction will occur. Your pedals will also be 19mm closer to the ground. That's not insignificant.
Bikes are designed with different geometry when 650b wheels are intended to be used. You might consider doing some more research before proceeding.
That's only if he uses the exact same tire width as before. Going from 700x23C to 650x42B would render the steering/handling/pedal strike thing moot, as the wheel diameter would stay the same.