I came home from school one day and my bedroom was a den with a loveseat, couch, TV, and mom's sewing stuff (no warning), and the former den was now my room but different. She hadn't mentioned it so it was a bit of a surprise, but whatever. Mom said she'd boxed up all my stuff for me to go through and that made me feel better---nothing had been thrown away and I could take what I wanted. I took some things with me to my apartment, took some things (yearbooks, etc) up to the attic because I didn't have room for them yet but didn't want to get rid of them), and 9 months later, I came home from school and my room was an office, lol. It's the natural progression of things. It didn't cross my mind to get upset; it was her house and I didn't want to live there either; I had my own life in another state at that time (age 20). However, I not only always felt welcome (after our very rough time from 18-19, from almost 20 on, I have always felt very welcome), I also knew mom needed me and wanted me around. I wasn't homeless or orphaned just because my room was changed, and I never felt that way.
If your daughter was 18 I'd feel for her a little bit, but at 23, um......NO.
She doesn't know 1 friend of hers that doesn't have their own room at their parents? Perhaps all those parents are still in the bigger house they grew up in and they just haven't moved so they haven't changed anything? She has her own apartment, and KUDOS to her for that, that's great. But that means that she is a welcome and loved guest in your home once a week, not a child who's still living with you. And it's not like you changed a room that she grew up in. You moved when she went to college, to a different house. With a different floor plan. Downsized. That's the natural progression of things unless mom and dad stay in the same house for 200 years. (Our children are 5 and 2, but we already know we will be downsizing when the youngest goes to college. An older couple does not NEED 5 rooms/4 baths to clean. Downsizing makes sense on a lot of levels.
ETA: It's your house, decorate it how you like, but I wouldn't see a problem with just a couple things to make her feel "at home" (I change the pictures out on the dresser in the guest room depending on who's coming in, lol.....photos of dad's family and a pic of the 2 of us when I was a kid when he comes to town, photos of mom and I when I was a kid and group photos of her side of the family when she comes, and next week my brother is coming and I've already switched the pictures to be their latest family photo and a couple of their kids piled on the couch with my kids.....hilarious pictures of my friends and I when we were "young" when they come to visit). They're all already framed, it's just a matter of what I pull out and set on the dresser....and I switch books around to peoples' particular interests in case they want to grab a book off the nightstand to read before bed, stock the house with a couple things I know they like to eat, choose flowers according to what they like, that kind of thing for guests.....so I guess while not making things permanent so that it's always that way (letting her decorate walls, bedding, etc) letting her know you still know her and think of her is good. Maybe she's just having a little difficulty taking that next step.