Hi DanielNJ:
Unless money is extremely tight, or if you’re just a fan of being a mattress “mad scientist”, I’d recommend starting from scratch and not reusing your innerspring unit. It will have gone through 12 years of fatigue and the innerspring unit will have taken a bit of a set. Also, the nonwoven material covering the springs will have become worn, especially along the glue lines, and you’ll notice some tearing and fatigue in these areas. The biggest concern I’d have would be for a healthy sleep environment, and your mattress is like an accordian, pushing air out and sucking things in with every movement. There will be some gross stuff embedded in your innerspring unit.
But with that being said, if you wish to continue…
Your idea of replacing all of the padding layers with latex would be a simpler and better option. You would have quite a few references online of popular combinations that tend to work well together, and latex is a much more durable product than your typical polyfoam.
If you’re attempting to replicate the polyfoam layers within your old mattress (which I wouldn’t want to do because the foam itself wasn’t particularly high in quality (density), you’d need to know the density and ILD of each of those foam layers, along with the exact type of foam to match things like the Support Factor, Dynamic Fatigue, Resilience, Tear Strength, Hysteresis, etc. Most likely you won’t be able to find the exact foam layers (technology and specs change), so you’re now left to find something that you “think” feels like your old foam. You can’t compare to the current foam layers, because they feel drastically different now from when they were initially placed within your mattress. This now puts you in the category of a mattress designer, which is a skillset very few people in American have, and you’re attempting to find different layers of polyfoam and be able to predict how they will work in combination relative to what you’re attempting to replicate. There are a lot of variables here, so you can see why I don’t recommend this option. Working with latex would be much more desirable, on many levels.
No all mattresses use convoluted foams, but they can offer a different feel and compression modulus as compared to a similar version of the same foam in a flat sheet. Like flat sheet polyfoam, look for higher density versions. And they can help to flow more air in areas where they aren’t compressed.
There is an excellent thread here on the site about DIY mattresses that you might like to read.
Also, there are some excellent latex component suppliers on this site who provide good advice on latex combinations that tend to work well together.
Good luck!