To me: I don't think it's a "deserve" issue. I think it's a usefulness issue.
If it's creating major problems keeping the kids where they are OR you want the eldest to always feel they have a "room" to come home to over college breaks... I'd change now. But do the "all in one" change. So that the eldest has "their" room for the next 5 years set, and the youngest ones aren't shuffling about constantly.
If it's not creating problems, and you want the eldest not to come home over breaks OR set up the smallest room as a "guest" room (aka where they can stay over breaks, as well as house guests)... wait a year and switch everything around then.
If you're doing it NOW, frame it that you want them to have a room of their own to come home to over breaks and have it be THEIR room. So you want to switch now, so that they can lay "claim" to it/ decorate/ have it be personal. Rather than just go off to college and come home to a room that doesn't feel like "theirs"