Not sure to follow. With VTK_LEGACY_REMOVE=OFF, there should be no errors and the results should be the same. There should be warnings though.
If they are not overriding, then results can’t be the same.
If you want to take a look into the specifics of my implementation, it’s here https://gitlab.kitware.com/vtk/vtk/merge_requests/5915, especially 1a56e913efbf778c068eb3420ae8b82e8fb5a5c6