Hi owenk,
Unfortunately, and like many of the members here that have been in a similar situation, you are in a somewhat difficult position where you need to exchange a mattress at a store where there may not be any particularly good quality/value options available to you.
There are some suggestions and ideas in post #2 here about the two main strategies that you can use that can help you make the best of a difficult situation. There is also more in post #13 here about the most important parts of the “value” of a mattress purchase or exchange that may be helpful as well.
You are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved for anyone else to be able to predict whether any specific mattress will be a good match for you based on specs (either yours or a mattress) or “theory at a distance” (see mattress firmness/comfort levels in post #2 here). The only way to know whether any mattress is a good “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) is based on your own careful testing (hopefully using the testing guidelines in the tutorial post) or based on your own actual sleeping experience. There is also more about the different ways to choose a mattress that can help you identify and minimize the risks involved in each of them in post #2 here.
I would also avoid any mattress that has thicker layers of lower quality materials in the comfort layers (such as most of the pillowtops made by the major brands) since they will almost certainly be a weak link in the mattress. Lower quality materials will tend to soften and break down and lead to the loss of comfort and support much too quickly relative to the price you paid and unless you can confirm the quality/durability of the materials in any mattress you are considering (see this article) I would tend to avoid pillowtops or any mattress that has thicker layers of either lower quality or unknown foams in a mattress.
Phoenix