One imagines that Robert E. Lee accepting service with the Northern Army would be the lynchpin of more than one Union Victory timeline - if Winston Spencer Churchill can spare time from acting as the United States' worth adversary, he'd almost certainly seize on this particular point of divergence (this might well be one of his more poignant works; heck, he might even have time to turn it into a full-fledged novel after his downfall, assuming the USA & Germany don't demand his execution).
On a more cheerful note, it amuses me to imagine the SOUTHERN VICTORY version of Professor Turtledove becoming famous for his factual works of history while Ken Burns becomes renowned for his command of Alternate History!