Keep old vertices index values after updating mesh

Dear all,

Let’s say I have a simple mesh and apply some transformation on it. Then, I call the line:

mesh.update()

As far as I understand, when the mesh is updated, the same happens to the vertices index, right? Is there a way to keep the old index of the vertices from the original mesh?
Maybe my explanation is a bit confusing, but the scenario is the following: I have a dense mesh m1 and I remesh it to a sparse one m2. But I want to use the vertices index from m1, to know the correspondence in m2. But when the mesh is updated, I lose the original vertex index, which is updated to a new one. Is there a way to keep this index? During the remesh, some vertex are deleted, but it does not matter, I want to know the relationship between the ones that remains. Any suggestions?

Thanks in advance!