Hi Curious1,
Yes it will … but how much it will extend it depends on the type and firmness of the topper and on the quality and durability of the materials in your mattress.
Your mattress is quite old so it wouldn’t surprise me if it had also softened under the heavier parts of your body or developed impressions even with a topper but if your mattress doesn’t have any soft spots or visible impressions across the sleeping surface and is still in good condition and the only issue is that it’s too firm by itself then it would generally be fine to just replace the topper.
If you do decide to just replace the topper … some of the better online sources for toppers I’m aware of listed in the component post here.
Phoenix