Hi RJ29,
As PapaMike mentioned … you can either exchange or return the mattress once only.
Yes … the support core in both firmness options is the same and it’s just the top layers that are softer.
If your mattress is too firm then a topper can be a good option to add some additional softness and pressure relief although there is always some risk and uncertainty involved in adding a topper if you haven’t tested the combination in person because the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice on any specific mattress.
If a mattress/topper combination turns out to be a good “match” for you in terms of PPP then it also has the advantage of being able to replace just the topper without replacing the entire mattress if it softens or breaks down before the upper foam layers in the mattress (the upper layers or a sleeping system tend to soften or break down before the deeper layers) or if your needs or preferences change over time and a topper can also help extend the useful life of the mattress underneath it as well.
The best use for a topper is to provide some additional softness and pressure relief to a mattress that is too firm and it’s much more difficult to firm up a mattress that is too soft because this would normally require identifying which layers in your mattress are too soft and then removing and replacing either comfort layers that are too thick and/or soft or a support core that is too soft.
If you add a firmer topper to a mattress that is too soft then the comfort layers (the firmer topper) may end up being too firm to be comfortable for you to sleep on and the topper will usually still “bend” or compress into the softer layers underneath it so you can end up with comfort layers that are too firm and support or transition layers underneath it (that were meant to be softer comfort layers) that are too soft to provide suitable support under the topper.
If you do decide to add a topper then there is more information about how to choose a topper in post #2 here and the topper guidelines it links to about firmness and thickness which along with a conversation with a reliable supplier that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. It also includes a link to a list of some of the better online sources for toppers I’m aware of as well and a link to the suppliers that have good exchange/return policies as well.
Phoenix