Nixon swept the entire South by huge margins in 1972, and so I do think it was rather impressive for Carter to win nearly all of it back, even if temporarily. I think the change started in 1964 when LBJ lost 5 Deep South states to Goldwater in what was clearly a reaction to his promoting the Civil Rights Bill and civil rights in general. Nixon followed a "Southern strategy" in 1968 and carried both Carolinas, Virginia, Florida, Tennessee, and Kentucky, while leaving the rest to Wallace (Humphrey did carry, barely, Texas). So I do think the region was already moving Republican and Carter's win was clearly a temporary break in that gradual shift. And so I disagree that the Reagan shift in 1980 wasn't significant. Clinton's recovery was also clearly temporary as well, as Gore lost every Southern state in 2000, including his home state of Tennessee.