The current ACC divisions are
Atlantic - FSU, NC St., Wake, Clemson, BC, Maryland
Coastal - Va Tech, Miami, UVA, Ga Tech, UNC, Duke
The ACC is losing Maryland and gaining Syracuse, Pittsburgh, and Louisville
I think it will try to keep the three new teams together and put them in the Atlantic with FSU, BC, Clemson and Wake or NC St. and just move the other one to the Coastal. That seems to make the most sense to me, but I don't really know who the big rivals are and I imagine they will try their best to keep them together (since you will now have 6 games against your own division it makes it harder to keep a rival in the other division without going to 9 conf. games).