Hi Joefox,
If you buy a soft memory foam topper then it will soften up the mattress and you will sink into the mattress a little deeper than you are now yes.
If the only issue with a mattress is that it is too firm and there are no soft spots or sagging in the mattress then a good quality topper can certainly be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know whether a specific mattress/topper combination is a good “match” for you in terms of PPP (Posture and alignment, Pressure relief, and your Personal preferences) is based on your own careful testing or personal experience on the combination. If you can’t test the combination in person then there is always some risk and uncertainty involved in adding a topper because the specifics of 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.
There is more information about choosing a topper in post #2 here and the topper guidelines it links to which along with a conversation with a reliable and knowledgeable 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 and a link to the online suppliers that have good exchange/return policies as well.
a 2" topper would certainly be a fairly “average” choice that will generally provide a “little to a fair bit” of additional softness and pressure relief to your sleeping system.
If a mattress/topper combination turns out to be a good “match” for you in terms of "comfort and 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 (which is very likely because the upper layers of 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 which in the case of your Serta may also be helpful because it uses some relatively low quality/density materials that can soften or break down more quickly than the higher quality/density materials that I would normally suggest.
No … I would suggest using a topper on top of your mattress and not taking your mattress apart. The “cool blue” memory foam in your Genius only has limited benefits in terms of temperature regulation anyway and there are many other gel memory foam toppers (or even some non gel memory foam toppers) that would be similar to the memory foam in your mattress in terms of sleeping temperature. Having said that … memory foam or gel memory foam will tend to sleep warmer than other more breathable types of foam materials and softer comfort layers that allow you to sink into the mattress more deeply can also be more insulating and sleep warmer than firmer comfort layers as well. There is more information about gel memory foam and other types of gel materials in post #2 here.
It’s not possible to quantify the sleeping temperature of a mattress for any particular person with any real accuracy because there are so many variables involved including the type of mattress protector and the sheets and bedding that you use (which in many cases can have just as significant an effect on temperature as the type of foam in a mattress) and on where you are in the “oven to iceberg” range and because there is no standardized testing for temperature regulation with different combinations of materials but there is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a neutral and more comfortable temperature range.
Any comfort exchange would be provided by a retailer that sells a mattress not by the manufacturer of the mattress. They are generally for a relatively short period of time that is only long enough to decide whether your mattress is a suitable choice in terms of firmness and PPP. The manufacturer is the one that provides the warranty coverage that would apply to your mattress but warranties only cover manufacturing defectsand don’t cover any “comfort” issues. A good retailer that you purchase from will generally work with you to expedite any warranty claim.
Having said that … when you purchase a used mattress then there would be no comfort exchange and there also wouldn’t be any warranty coverage on your mattress so buying a used mattress is very much “buyer beware” (see post #2 here).
Phoenix