Hi rvsarch,
This would be typical for memory foam as it will soften over time under the heavier parts of the body especially (virtual impressions) more than sag (visible impressions) and will also lose some of its “memory”.
There is more information in post #9 here about the different ways that one mattress can “match” or “approximate” another one. Every layer and component in a mattress (including the cover and any quilting materials or mattress pads) will affect the feel and performance of every other layer and component and the mattress “as a whole” so unless you are able to “match” every layer with exactly the same type of material that has all the same properties with the same layer thickness and the same layer firmness you won’t be able to “match” your original Tempurpedic or your memory foam/latex replacement.
Latex and memory foam are completely different materials with very different properties so using a latex base layer that is very different from the original polyfoam base layer in your Tempurpedic would already make it impossible to “duplicate” your original Tempurpedic even if you were able to find a memory foam layer that had identical properties to the memory foam in your original Tempurpedic.
In addition to this there is a very wide range of different memory foam formulations with different properties including density, response time, temperature sensitivity, breathability, and firmness. Many of the newer memory foam formulations or gel memory foam formulations are more breathable and have a faster response time (less “memory”) than older formulations because of consumer complaints about heat and about the “stuck in sand” feel that can go with some memory foam formulations that have a slower response time. There is more about the different properties that can be formulated into of different types of memory foam (gel or non gel) in post #9 here and in post #8 here.
In other words it won’t be possible to “duplicate” your original Tempurpedic or your latex/memory foam replacement using different types or formulations of materials.
The good news though is that you won’t have to duplicate it anyway because all that really matters is that any mattress or combination of materials and components that you sleep on is a good “match” for your own specific needs and preferences in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) and whether it matches another mattress or combination of materials or not isn’t particularly important (just like your original change from a polyfoam core to a latex core was an improvement over your original Tempurpedic even though it was different).
The closest you could probably come to your current latex/memory foam combination would be to replace your current Tempurpedic memory foam layer with a 3" layer of a different type of 5 lb memory foam that was “somewhat similar” to your Tempurpedic layer.
The better online sources for replacement layers and components I’m aware of are listed in post #4 here and a conversation with a reliable and knowledgeable supplier can also 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 so you can choose the type, thickness, and firmness for a memory foam topper that has the least possible risk and the best chance for success. A good exchange/return policy can also reduce the risk of an online purchase so I would make sure you are comfortable with the options you have available after a purchase just in case the memory foam layer you choose doesn’t turn out as well as you hoped for.
Different people with different body types, sleeping styles, and individual preferences and sensitivities can also interact with the same mattress very differently so they can have very different opinions about how two mattresses compare so I would always keep in mind that the only reliable way to know for certain how two mattresses would compare for you in terms of how they “feel” or in terms of firmness or PPP would be based on your own careful testing or your actual sleeping experience and that some degree of trial and error will always be part of any DIY mattress project.
I would keep in mind that the firm latex that you are using in your base layer will feel very different from the softer latex that is generally used in comfort layers so it may be a good idea to test out some mattresses in local stores that have top layers of latex that are softer than your current base layer to see how they feel to you and to get a general sense of the firmness level that you tend to prefer.
If the memory foam has softened under the heavier parts of your body then it could certainly have some negative effect on your alignment over the course of the night although it would probably be less on the bottom than on the top. Since you have separate layers anyway … if you are considering using a latex comfort layer instead of a memory foam comfort layer I would tend to use the latex on top of your current latex base layer and remove the memory foam completely. Since you appear to like firmer mattresses if you do go in this direction I would probably start with a 3" layer of latex that was somewhere in a medium firmness range and then if this was too firm you could either add an additional softer layer on top of it or replace the medium layer with a softer one. Again though I would keep in mind that latex and memory foam are very different materials with very different properties so it’s difficult to use one as a reference point or “blueprint” for the other. There is more about some of the differences between memory foam and latex in post #2 here.
Phoenix