What are the requirements for a Combat Action Badge versus a Combat Medical Badge?

I wouldn’t consider it a no no. The CAB, while it was originally intended for individuals who were not eligible for the CMB/CIB, can be awarded to anyone that engages or is engaged by the enemy. This of course includes indirect fire. It’s not common, but not unheard of to have both. I personally have both the CAB and the CMB. It could be argued as you said that you performed your medical duties while being engaged by the enemy. As for it being retroactively changed I’m unsure if this is possible. I do know that you can’t be awarded both for the same qualifying events. In my opinion if you feel that you should have been awarded the CMB instead or better yet, if you have a different qualifying event that would be even better. I don’t know who you talked to at HRC, but that isn’t soldier care. You might request to talk to someone else.

After reading AR 600-8-22, it appears that if a medic is assigned to a combat arms unit, they are not eligible for the CAB. They have to perform their medical duties satisfactorily while actively engaged in order to be awarded the CMB. Depending on who interprets that, a medic being in a firefight does not qualify for any type of badge assuming they are attached to a combat arms unit.

8–7. Combat Medical Badge
(6) The sole criterion that qualifies medical personnel for award of the CMB is to be assigned or attached to an
infantry, combat arms, or combat aviation unit engaged in active ground combat

8–8. Combat Action Badge
"(4) Soldier must not be assigned or attached to a unit that would qualify the Soldier for the CIB and/or CMB. For
example, an infantryman (MOS 11B) assigned to Corps staff is eligible for award of the CAB. However, an
infantryman assigned to an infantry BN is not eligible for award of the CAB."

If you want an official reading and answer from the Army on medals awarded and criteria used. You can submit your case to the Army Board on Correction of Military records. I am not sure this will help you now for a medal awarded in 2009 but submission is free and if it is really bothering you, why not spend a few minutes and fill out the form. The downside is of course is I am not sure they can do award swaps, so it might be if they agree with you........you lose the CAB without a replacement. Maybe they can........I have no idea and it is your risk to take.