You might get a little bit of an improvement in standover height from a 650 conversion, and it sounds as if you want only a little. You'll drop standover height by 19mm (about 3/4") from the the wheel switch, but you'll probably give some of that back since many 650b tires are taller (in cross section) than the typical 700c road tire. Don't forget to check tire prices when you are figuring expenses. Waiting for a good mixte might be a more reliable course to take.
Good luck, either way.