I turned my oldest sons room into a guest bedroom when he left for college, new furniture and all! It needed it, and we needed another guest room. I wanted him to still feel it was his room, so left a few memorabilia on the dresser and chest. A few other well selected photos. He lived there for 8 months after college. We refer to it as his room, and it really will always be.
We’ve not changed the youngest room. Partly because we don’t need it as a guest room and partly we don’t want to spend the money yet. I imagine he will be living with us after college. At that point we may make it a nicer, more grown up room for him.