Hi gmgdwnchblck3,
Unfortunately you are in a similar position to other forum members who have purchased a lower quality (or sometimes higher quality but poor value) mattress and are only able to exchange for other options that are available in the store.
Post #66 here and the links it contains would be well worth reading.
Your “best” course of action would depend on which mattresses are available in the store you made your purchase and if you only have access to Serta then this makes your choices much more difficult.
The goal is always to minimize any lower quality foams in the upper layers of your mattress which are generally the 'weak link" in every mattress. If there are no mattresses available that are suitable for your needs and preferences … then the “best” course of action is generally to exchange for a mattress that us very firm and uses the least possible amount of lower quality materials in the comfort layers and then add a topper or your choice that is higher quality for the pressure relief you need.
This is especially important when you are in higher weight ranges and all materials will be less durable than they would for those who are lighter.
The iComfort Prodigy and the Renewal Refined are both in the softer end of the iComfort lineup and already have thick comfort layers so a topper on top of these would make the comfort layers even thicker than they are and would be very risky ion terms of alignment … especially with your weights. I would look for a much firmer mattress with the least possible amount of questionable materials in the upper layers and then add the comfort layer you need with a topper rather than on top of one that may already be too thick and soft for you.
I should also mention that the iSeries enjoyment has one of the thinnest comfort layers in the lineup (only 2" of the gel over the innerspring) and is very firm so it may also be a good candidate for a topper to fine tune it for the pressure relief and comfort you need rather than exchanging it for a higher priced mattress that has thicker layers of somewhat questionable materials. this is assuming of course that the mattress keeps you in good alignment and the only issue is pressure relief and comfort. With a topper you can control the quality of the materials in the top layer of your sleeping system which will be the first to soften and degrade. It will also lengthen the life of the foam below it. If it does soften faster than the rest of the mattress … all you have to do is replace the topper rather than the whole mattress.
Phoenix