You essentially have to prevent American entry into the First World War, or any major war during the first half of the twentieth-century, for this to happen. And even this wouldn't stymie the Great Migration entirely; much of it was predicated on the emergence of the Second Klan in the South. You'd probably still end up with a Lesser Migration regardless.
Even considering this, though, there was always gonna be a post-Reconstruction racist backlash (i.e. Jim Crow), meaning that there were always gonna be affected people fleeing for the North too (especially when they're incentivized to do so by the labor shortage & good-paying factory jobs available for them in the North to take if they do so).
