I don't think conversions are that uncommon. Depends on the frame and what tyres you're planning to run?
Assuming you are running rim brakes, you encounter 2 problems:
1) Calipers might run out of upwards adjustment (might have to buy different calipers)
2) Pads move further away from the stud, decreasing mechanical advantage. Braking might become spongy.
With disk brakes I'm pretty sure you can fit 700cx25 into some 26er frames.
Don't know why anyone would do that, but hey, I don't judge. What happens behind closed bike shed doors is not my business.