On a practical level, I would expect what happened at the other neighborhood school to happen -- the school will still be majority African-American, but white children will be much closer to half of the school than they are now.

I will note that this DIDN'T happen at a third nearby magnet-that-used-to-be-neighborhood-school, but I think that has to do with a historical reluctance of white people to go into that neighborhood at all (even to drive through it as a shortcut), whereas the neighborhood my kid's school is in has a large white population. (The school is not currently demographically representative of the neighborhood where it is located, or even of all the neighborhoods from which it feeds; most white parents are sending their kids to private schools or to other magnets.)