Can a 70B serve in a 72D position?

Lt,
In the USAR it appears that they do not mind but ensure that you complete the AMEDD requirements. I had worked a couple years as an MP Officer,Combat Engineer, but for the most paret I tried to stay with the medical units. You will find that the other branches do not know what to do with you, and since you are not their branch you will get to do what their officers do not want to do. The lower enlisted medical folks generally have at least an Associated so communications is a heck of a lot easier. If you can help it do not deploy with Civil Affairs Worst mistake I ever made was going to Iraq with an CA commander why had no couth.
If you know what best for you the person, soldier, do not switch AOC's. All AMEDD advanced courses are offered at Fort Sam (San Antonio, Tx).

If you aren’t branch/duty qualified in that AOC and for that position, then I don’t know how you’d be able to serve with or without commander approval. Maybe I missed part of the question

I don’t know why a Commander would do this since it doesn’t help his unit status and MOS/AOC qualification something that is intensively managed in the Reserve Component. Are you considering an AOC change?