Hi dfarr67,
Like many of the members that have come 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 that I would normally suggest avoiding and 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.
Unfortunately the Sears comfort exchange is 60 days (365 days in Canada) and you have a warranty exchange rather than a comfort exchange so I don’t think it would be likely that they would give you a refund but it’s always worth a try.
There aren’t any mattresses there that I would choose and they also don’t provide the information that I would need to make any meaningful comments about any of their mattresses.
If I was in your shoes and they can’t provide the information that you would need to make an informed choice about any of their mattresses … I would probably lean in the direction of choosing a firm mattress that had the least amount of “unknown” or lower quality foam (which would have less risk of premature foam softening and breakdown) and then add a high quality softer topper to provide the comfort and pressure relief that you will need. While choosing a suitable topper can be somewhat uncertain because you won’t know for sure whether it will be a good match for you in combination with the mattress you choose until you sleep on it … I would personally rather take this risk and end up with a more durable choice than the risk of choosing another mattress that will soften, break down, or impress much too quickly relative to the price you paid. You can always soften up a mattress that is too firm with a topper but it’s very difficult to “fix” a mattress that is too soft or has developed soft spots or impressions without removing and replacing the foam layers that have softened or are sagging (see post #4 here) and if the visible impressions aren’t deep enough you may not qualify for another warranty exchange even though you may not be able to sleep on the mattress any longer (see post #174 here about mattress warranties).
If you do decide to go in this direction then post #2 here and the topper guidelines it links to can help you use your sleeping experience on the mattress as a guideline to choose the type, firmness, and thickness for a topper that would have the best chance of success.
Phoenix