Is there an Army get protecting soldiers from being demoted while on Med Board?

Of course not. A soldier can be processed for chapter at the same time as a med board.
Regulation is AR 635 - 40

While in medboard you are still subject to all UCMJ and army regulations. A medboard does not provide you any special protections. In fact, if you were to say pop hot on a urine analysis, you could be chaptered and be removed from the IDES process, thus being left to make your own claim with the VA once you're out, assuming of course your discharge allows that benefit. Someone in the IDES process is still subject to their command.

BY demoted are you asking CPL to SPC with is a lateral transfer ?, if you are saying pfc/pvt and there are grounds for reduction the you are out of luck.

Army Regulation 635–40