I think it is probably something to debate.
In some ways it seems obvious to do that, but it's quite a lot of hassle, and then the new uni person is coming back to a new room etc which can be quite unsettling. I know I felt the first holiday at uni really unsettling-I felt I didn't belong properly at either place. I found the second term really difficult, much harder than the first, for that reason.
And sometimes they can feel pushed out. My dsis said right the way through A-levels that as soon as she was off to uni (doing medicine with 48 week years too) that she was expecting my dbro to swap rooms (when it came to it he didn't as he thought it would be too much hassle) . Her room was probably more than twice the size of either his or mine, mine was the smallest) and said she wouldn't care a bit.
When it actually came to her going she suddenly got very possessive about her room, and struggled with finding anything had altered over the time away, even little things. She saw any change as a sign she was being pushed out, even though over the uni years she rarely spent more than 2 weeks a year at my parents' home. She even struggled with when she'd left home and was married with my parents referring to it as the guest room.
I think for me it slightly depends on the discrepancy in size or of facilities, I can see having an en suite could be seen as a big advantage.
If it's slightly bigger, it wouldn't be worth the hassle.
But it also depends on their relationship and how they feel about each other.
I would have deeply resented my dbro getting it, because there was a history of my brother getting things of mine before I'd finished with them, or identical things I'd bought for myself or got as presents he got given because my parents could "see how much I liked something and thought he'd like one too." If your dd feels he's the favourite anyway (not saying he is, but often children do think that sort of thing with or without justification) then that would add to the feeling.
I've got my oldest moving off to uni next year. She doesn't have the biggest room, dd2 does. It's bigger than ds' though, so what I've said is he can put his drum kit, which presently takes up half his room, during term time in there. It means that he will have more space, but she can still come back to "her" room.
I think also the people above who said that they waited/will wait to check their dc have settled at uni first have a good thought. I said I was unsettled by the first holiday. By the summer holiday I was much more secure and it wouldn't have effected me changing my room.