If I am reclassing to a new MOS whose course may be longer than 20 weeks, do I have to move my entire family or can I go as a geo bachelor?

A course that is 23 weeks or longer will be s PCS move. A lot of these PCS length courses, especially medical courses, are multiphase with the first portion completed TDY enroute to the second portion.
When you go to your levy brief you'll be given several options of what to do with your family. You aren't required to bring them.

In my experience, any course longer than 6 months is counted as a PCS. 20 weeks being under that mark would imply that it would be a TDY enroute or in-return. Your best bet is to call your career counselor, or contact the school house for the new MOS. You could also go to the personnel office that cuts your orders. Career counselor is probably the most quickly able to find that answer for you.

you can go as a Geo bachelor, a cpl in my platoon med re-classed and sent his family home until he finishes his ait and goes to his next duty station