Hi mark_in_sf,
Congratulations on your new mattress first of all 
You certainly made a great quality/value choice.
The mattress has three different 3" 2 lb polyfoam layers in different firmness levels that can be rearranged and configured in 6 different ways (although some configurations like soft on the bottom, medium in the middle, and firm on top would be much less common). In addition to this each of the combinations can be used with either the softer side of the zippered cover or the firmer side of the cover on top (each side has a different quilting pattern which makes it firmer or softer) so that gives you a total of 12 different configurations. They don’t offer a layer exchange because there are already many possible configurations and to keep the costs down (layer exchange options add to the cost of a mattress).
If you ever did want to replace a layer then you would just need to buy a 3" layer using the the type of material (polyfoam, memory foam, or latex) and firmness level that you prefer, zip open the cover, and remove and replace the old layer with the new one and zip the cover back up. If you ever decide to go in this direction then some of the better online sources I’m aware of for component layers are listed in the component post here.
I’m looking forward to your comments and feedback once you receive it.
Phoenix