Hi Lanesville,
As you can see from the guidelines here … Simmons along with the other major brands are among the choices I would suggest avoiding and are not the type of quality and value that I would normally suggest even considering.
You are in a difficult position because most of the chain stores like Sit-n-Sleep don’t usually have any mattresses that I would consider to be good options or value for either a purchase or an exchange. The only brand they carry that I would tend to consider as a purchase is Sherwood which at least might provide you with better options than most chain stores usually have available.
There are other forum members though that have been in a similar situation (needing to make an exchange with a major brand mattress or through a chain store) and post #2 here and the other posts it links to will give you some insights about how to make the best of a difficult situation.
As you will see … if you exchange for another major brand then the option I would lean towards would be to exchange for a very firm mattress that has the least possible amount of lower quality polyfoam in the comfort layers (which is difficult to find because they all tend to have too much) and then add a high quality topper to add your own comfort layer. This will generally give you the best possible quality and durability. This or one of the Sherwoods would probably be the best choice in your circumstances.
I would also make sure you read post #1 here which along with the information it links to is the most important post on the forum and will provide you with a lot of basic information that will also be helpful even though you are exchanging a mattress rather than starting the search to purchase one from the beginning.
Phoenix