I recently bought a 2015 Giant Anthem 3 27.5" full suspension mountain bike to build into an electric bike, however, all my other bikes (road bike, other electric bike) have 700c/29er wheels, and I think 700C/29er wheels might be better for this particular build. My new 27.5"/650B mountain bike is running relatively thick 2.25" mountain bike tires, and it has just over a half-inch of tire clearance on the front fork as well as the rear swingarm/triangle thing. I would like to change out the 27.5" wheels for 700C/29er wheels with smaller tires (say, 40C/1.75" tires). My working theory is that it would fit just fine, because the increase in wheel diameter would be offset by the decrease in tire diameter. Is this a workable, advisable change? Could I use the same brake rotors (160mm discs front and rear) on the 700C/29er wheels? Assuming that 700C/29er wheels would fit on my 27.5" frame, should I potentially avoid adding fenders due to the tighter tire clearance?
The only way is to try it and find out. The great thing about disc wheels is that the brakes are in the same position regardless of rim size, so if you put the wheel in and it physically fits (i.e. it goes in the frame with enough clearance) and for the rear it has the right cassette, you're good to go (possibly with some minor adjustment to the brakes). If the outside diameter of the wheel with the tire on shrinks versus your 650b wheels with the 2.25" tires, it should work fine. If it grows, you may have to worry about clearance. You can check this by putting the wheels on top of each other.
That being said, reducing tire size may not work so well for actual mountain biking, so if you are going to take it mountain biking rather than electric biking on the road, you may want to stick with the bigger 650b tires.