The older white folks died off and were replaced by black families. LI is the most segregated are in the country, by choice of all parties. Black LIers choose to invest their home buying dollars in "black" towns. It has been that way since the forties. Gordon Heights in Suffolk was created specifically to be a black Levittown.
Hempstead hit the tipping point in the sixties and the town has deteriorated steadily since then with the Carter years hitting particularly hard.
Have a happy...
The amount of segregation on LI is puzzling in that, even in NJ, you still have noticeable populations of middle class and upper class blacks and Asians in the "white" towns. LI still has "Catholic towns" (re: guido towns) and "Jewish towns" in my understanding, where folks freak if someone from outside those tribes move in (see Valley Stream, which was largely Eye-Tie in my youth, but has become heavily Hispanic and black over the past decade). You are right in that it cuts both ways.
On a more positive note, have a Happy Thanksgiving!!!