Hi jduub,
You may have seen this already but just in case you haven’t, post #2 here has more about mattress surgery and links to other posts and sources of information that may also be helpful.
I would test the foam you are planning to keep to make sure that there are no soft spots or sagging in the layer. You can test for sagging by putting a layer on the floor and putting a straight edge or string across the surface and you can test for soft spots by pressing in different areas by hand or by using a heavier object such as a bowling ball or bucket of water and putting it on different areas of the layer to see if it sinks in the same amount in all the areas.
Once you have decided on which layers to keep and which to replace then you can try the mattress with just the layers you are keeping to see how it feels to you and then post #2 here and the topper guidelines it links to can help you use your actual experience as a reference point to help you decide on the type, thickness, and firmness of the layers you decide to add to the mattress.
I’m looking forward to finding out how everything works out 
Phoenix