I think it's okay since your oldest will still have a room to come back to. I'm cleap though, so I'd be tempted to tell her she can swap as long as there are no decorating costs for you!
My sister and I continued to share a room when my brother left home, but if one of us had taken his room he wouldn't have had a place at all. At the time I was irritated that he wasn't home, but still had a room - now I can see how that would have been weird for him. Most college kids come "home" occasionally and don't really have a place to call their own home until later - especially if they live in a dorm.