Question: Changing a Battle Command to a single spell

Hi all! Back to giving this a try now that I'm quarantined. A little update on where I am:

I've identified several commands I'm seeking to remove from my hack as they will be unused, and using the spaces where they once sat as brand new commands. The majority of them I feel fairly confident I could create as spells in FF3usME, and then just get the command to call that spell. (i.e.: I don't need Possess or SwdTech. Let's erase them both entirely and replace them both with "Scan" and "Charm" by just calling those spells.)
 
It was suggested before that the way to do it would be to copy the code of "Health" or "Shock" and just have it call a different spell ID. (just plopping down that code doesn't work though.) For example, I erased the "Possess" code at C2/17E5 and just replaced it with the "Health" command code found at C2/171E, but the character still seems to perform the "Possess" command. Then, I saw this quote from C-Dude:

Health and Shock have hardcoded animations based on their command IDs (GP Rain does too).  To get around that, you can change the command ID to #02 (MAGIC) in your block of code (by storing it in $b5) before executing a hit.

How do I change the command ID to #02 by storing it in $b5? I'm not familiar with this, and it seems to be the thing getting in the way of my success. Is it possible to do this with just a Hex Editor? So far I've only been hex editing and not doing any assembly.

Also, on a slightly-related note, could a command be programmed to have the same effect as a Smoke Bomb? My thief could have the "Flee" command if it were possible.