Also, I would say it's possible for people to change their minds, and remove their Neo-C coding, like how Kristin's abuela did. So implanting the cortical stack just allows for the option of choice.
Stacks are inserted at a year old (or probably upon creation in regards to clones), so I assume there's some sort of basic medical procedure that goes along with it. But the meths are switching sleeves through needlecasting, basically projecting their consciousness to their new sleeve. I'm unsure of the process that would initiate this, but that's the premise of how Rei was able to animate her clones the way that she did.