Hi Sleeping Organic,
Both of the mattresses that lynnfrbs1 was considering did have layers that are swappable, so this wasn’t an issue in this particular case.
Many people do enjoy the ability to be able to customize the feel of their product, not only directly after the time of purchase, but also after a long period of time in case their comfort preference changes.
You can see some comments about the pros and cons of a component mattress with a zip cover where you can choose the type and firmness of each layer vs a “finished mattresses” that has glued layers in post #15 here and post #2 here.
Phoenix