I believe a 650B is 584mm and a 26 is 559mm so the overall difference is 1" which means a 1/2" in radius. So depending and what kind of tire you're using and the particular I'd say maybe. Why do you want to do this?
As to the question about why... I have a FS that I don't ride that much due to location. I'm looking for a steel HT and was haeded towards a 29er or Bside. I came across a Indy Fab used frame and started thinking about doing a stiff setup with a carbon fork and an oversize front wheel. I was just wondering if there was any chance to swap front wheels between both bikes.