Hi pmsmith2032,
Mattress surgery would be used to remove and replace any materials or components in your mattress that have developed soft spots or are sagging. It would be useful as an alternative to throwing away the mattress because you could reuse some of the layers and components in the mattress such as the springs if they are still in good condition (which is often the case) and just replace the layers that are breaking down or sagging. You can change the firmness or feel of the mattress by adding softer or firmer layers and using different types of materials to replace the defective layers depending on the overall design and feel you want to end up with. There is more detailed information about mattress surgery in post #2 here and the other posts and sources of information that it links to.
A topper is only an effective solution if your mattress is still in good condition and there are no soft spots or sagging in the mattress and you just want to add some additional softness and pressure relief to your sleeping system. Using a topper on top of a mattress that is starting to sag or has soft spots under the heavier parts of the body such as the hips/pelvis (which yours probably does) isn’t particularly effective because the layers under the topper that are sagging or have soft spots can still compromise the support of the mattress.
In other words … a topper can only “fix” a mattress that is too firm but is otherwise in good condition while mattress surgery can “fix” a mattress that has defective layers and has lost it’s comfort and/or support (as long as the deeper support layers or components are still in good condition) although it can take some trial and error to find the combination of replacement materials that will be a good “match” for you in terms of comfort, firmness, and PPP.
Phoenix