Changing wheel size - what should I know?
• Swapping the wheels usually needs a little derailleur and brake adjustment, no matter what. You still might need to adjust the high and low limit. And, reposition the caliper, if  hydraulic. If mechanical, most have the pad adj knob, and barrel adjuster.

• Since you're coming from a hybrid wheel, going wider is an issue with the seatstays. But, if you are riding in thicker mud, a narrower rear tire is better. In fact, they way it'll have to work is ideal, with a wider front, skinnier rear. The width measurement does NOT include the knobs. They could rub, if wide enough. But, usually a deep chevron style tread is better for rear mud tires.

• The 650B is coming back into mainstream, after a long-abandoned experiment. So, you'll have more choices of wheel and tire. It will be $$ because of the exclusivity.

• Many wheel sets will have 100/135 spacing. Make sure you get non-thru axle hubs, just the 100x9 and 135x10 QR axles.

• Make sure you get the correct rotor mounting system. Mostly, there is center-lock, which uses a cassette lockring, and then the 6-bolt design. 

• The rotors will be in roughly the same position between both wheelsets. I do not recommend extensive lateral dishing of the rotor. It's good practice to be handy at repositioning the calipers. Even taking off and reinstalling the same wheelset can still rub, no matter how perfect the brakes were before.

• Don't worry about similar levels of cassette wear. Replace the chain more often, and use the gears a lot. Nine speed chains came down in price recently. If the chain is worn now, most definitely. But, this discrepancy will happen with prolonged use of one wheelset. If you use both wheels a lot, it won't happen as quick.

• Narrow mudgards obviously won't work as well with fat tires! There could be some splatter.