Literature: The Nephite/Lamanite contrast has always been part of Mormon’s text, but now he recasts their roles. The Nephites and Lamanites are still separate people, still in opposition to each other but occupying switched positions on the spectrum of righteousness. As the righteousness of the Nephites fades, the righteousness of the Lamanites increases.
This literary device allows Mormon to show the gospel’s continuation and overarching value. Even when the Nephites abandon the gospel, it is still true and blesses the righteous Lamanites. Their switch also explains why the Lamanites survive while the Nephite polity is destroyed.