Hi Elusiv,
You will know for certain when you open the cover but the odds are high that the top layers of foam or fiber are the underlying source of any impressions or soft spots.
A topper needs a firm even surface and if you add it over a mattress that has impressions or soft spots it will only bend into and follow the impressions or soft spots. There is more about “fixing” a mattress that has impressions or soft spots in post #4 here. If you remove the foam that is the cause of the issue and replace it with the latex (mattress surgery) then it could work well but of course the choice of thickness and firmness in the form layers is the key to success. There are some topper guidelines in post #4 here that may be helpful.
This would certainly be better than adding the topper over the side with the impressions but it will be very firm and once again the key is the thickness and firmness of the topper that would be most suitable for you.
If you make the most suitable topper choices then this could certainly extend the life of your mattress by a few years but if the topper choice isn’t suitable for you then it wouldn’t make a lot of difference because the mattress may no longer have any dips or soft spots but you may have a mattress that you couldn’t sleep on.
In theory it could certainly work well but in practice it would depend on the suitability of the topper/mattress combination for your body type and sleeping style.
Phoenix