Hi paisley,
You can also see some of my thoughts about Chiromatic in post #7 here (which I linked to a different and duplicate post in another one of my replies to you).
If they were making a custom design before you purchased it they would just make the new design from the beginning. If a manufacturer offers the option to make an adjustment after a purchase, then yes … they would open up the mattress, remove and replace layers or components, and then sew up the cover again. This is not something that every manufacturer even offers so if you were buying a mattress that you hadn’t tested then you would need to make sure that this was even a possibility.
Phoenix