What happens to my rank in medical school?

If it helps as a guide without citing specific regulation, a dear friend and colleague went to med school as a MAJP and is now a LTC. You will not be demoted for achieving more education. All the best.

Are you attending as part of one of the scholarship programs? HPSP, MDSSP? Or are you completing a Guard program? I ask, because then there is the potential to have your commissioned service cut in half. But I would check with your AMEDD recruiter, they will need to submit your Prior Service record and then they will determine your grade.

From what I see about the MDSSP, it looks to be based on the same principle as SMP for people pursuing their undergraduate. Meaning that you are not receiving a scholarship so you are not falling under Cadet Command in a not-for-credit pay status. It appears you would just continue serving in your current commission and performing drills while you go to school. That also means that your school time would count towards retirement as well, where ROTC or Cadet time, or USUHS would not.