I can see why most people are saying swap, if DD2's room is very small. Wonder if maybe it would be better to wait a bit though. Going away to Uni is an unpredictable time - you don't know if DD1 will settle in. I know people who dropped out and re-applied for a different uni/course. Unlikely, but I wouldn't want to disrupt her too much yet. Of course I don't know her.
I am the youngest and did not get older siblings rooms (much bigger) when they went to Uni - I did get the biggest room after a bit, but not immediately. I would have felt like I was pushing them out if they couldn't even use their own room in the holidays. Although if they'd kept it for years when not there and I was still in school, that would have been harsh.
Personally also, I had to bring all my stuff home in the holidays - we had to leave rooms completely empty so they could be used for conferences. I just think your DD1 is hardly out of the door - why not let her settle in and re-negotiate in the New Year. DD2 can always put some of her stuff in the room, or use it for sleepovers, during the term.
I don't really get the attitude of bending over backwards to accommodate children, then suddenly at 18 they have no say unless paying the rent and mortgage! I know you didn't say that OP, but I have seen it on MN. Crikey, give people a moment to get their bearings. If you're in full time education you can't necessarily pay the rent or mortage. It was strictly forbidden and enforced at my university that you were not allowed to have any part time job in term time.