What is different about working with an STL?
An STL describes surfaces as a mesh and doesn’t keep the editable structure of a source CAD model. Mesh-editing tools exist, but technical ability doesn’t mean the workflow is efficient or that the file will be a good basis for what comes next.
Resizing is a case I am willing to consider
Resizing is the main change I am prepared to make directly on an STL, and only after checking the file and the structure. A simple ring with a free area is nothing like a full eternity or a ring set all the way around.
For a significant change, compare it with a rebuild
When the head, the stones, the proportions or large areas need to change, mesh editing can become complex work that doesn’t pay off. Sometimes a new CAD model, faithful to the original, is the cleaner and more economical route.
A rebuild has added value
A clean CAD source gives the client an editable basis for future sizes, variations and further changes. That is why the model’s future life should be part of the decision.