You can always go for a 650B wheelset and change the brakes, while hanging on to the old wheelset and brakes if you ever want to change it back.

Depending on the tires you go with, it will drop the standover a bit. That's what I did with my 23" '73 World Voyageur. See http://www.flickr.com/photos/cpb282/

Whatever you do, good luck with it.