The idea of "white flight" equivalent to the Migration is overblown. Don't forget, OTL, blacks were fleeing not only racism & Jim Crow but actively being put back into functional indenture, & slavery, by labor laws & prisoner "sales". Whites weren't.
As for impact, how about culturally? You've buggered the history of blues, without blacks in St. Louis, Chicago, NYC, & elsewhere, bringing their music to white audiences (& mixing it). You've also buggered R&R pretty nicely, for the same reasons.
You may've butterflied the creation of NAACP. Maybe ACLU, too. You've wiped out the Harlem Renaissance.
You probably wiped out the Apollo Theatre & the Cotton Club; how many acts, black & white, owe their start to one or the other? (Not least, IIRC, Buddy Holley & The Crickets...)
So what does the U.S. look like without Big Band music, Western Swing, Elvis, & Buddy Holley?


