Actually, I can understand why DD1 doesn't want to give up her room - leaving home for the first time is scary and getting rid of 'her' room will be compounding these anxieties.
I do think she's being selfish, but for perhaps understandable reasons. Although she is an adult, at 18-19 plenty of people lack the ability to see the bigger picture.
Of course DD2 should move in to the bigger room, but I think doing it the minute she walks out the door is a little unfeeling if she's not happy about it.
I suspect that two or three weeks into term, when she's settled in a bit, she'll be much happier about the idea.
Also, why do the older siblings always get the bigger room? I was the youngest and had the biggest room in all our houses from the age of four up. I feel quite sorry for my brother now, he had the box room from age ten through to twenty three, when I left home...
Come to think of it, my mum turned my old room into a guest bedroom...with my brother still stuck in the box room! Poor sod. He did move into my room/the guest bedroom eventually though.