For one, Professor McGonagall was now Headmistress. She would likely be stricter than Dumbledore. Whereas he was perpetually aloof (or was he?), McGonagall would be privy to students breaking rules and sneaking around after hours. Also, instead of living in their House dormitories, the eighth years would share one common room with dorms in the seventh floor tower, near McGonagall’s office. While she and Ron would have their own separate rooms, everyone else would be rooming in pairs.