If the ACC were to ever implode, UVA would certainly go to the B1G. The UVA Board of Visitors would never allow their school to be a part of the SEC.
The UNC administration would feel the same way.
However, UNC fans would want the SEC overwhelmingly. They would want annual rivalry games with border states SC, Georgia, and, Tennessee, over OSU, UM, and, MSU. As much as I respect those programs, and, their histories, I have zero desire to venture to Columbus, Ann Arbor, or, East Lansing for games.
We have attendance issues now. If Carolina ever joined the B1G, I believe the fanbase would be in complete revolt, and, you'd REALLY see how empty our venues would be.