You can do the conversion the other way too, from a smaller wheel to a 650b. Rigid steel mountain bikes can make nice touring bikes, but if you want narrower slicks you have a much smaller diameter tire as a result. A lot of 26" wheel mountain bike frames can fit a 700c rim, but not with a 32 to 38mm wide tire. If you can get the brake setup to work a 650 wheelset is a nice option.