I think it's too soon. She needs to lead on any decisions about her bedroom.
You have 4 bedrooms upstairs and one other adult son? Are there any other children in your household? What are the other bedrooms used for?
When I went to uni, my bedroom stayed my bedroom. When I finished uni, I got a full time job in the sector that linked to my degree in my home city but it wasn't well paid and I had to move in with my parents again. I was heartily grateful for the home-base and the familiarity of my own bedroom at a time when so much was changing in my life.
In first year I was in halls so I came home for long holidays as we had to vacate halls out of term time (we had about 24 weeks of uni so that's 28 weeks at home). My sibling went to Oxford and was home for many months too during their 3 year degree (same as my uni, less than half the year in uni). My other two siblings had flat-shares throughout so they were back less but still used their childhood bedrooms for holidays and after uni.
I moved in with my boyfriend when we were 23 so I was only home for a couple of years but I don't know how I'd have felt if I was in a spare room or temporary room.
My mum's sister redecorated my cousins' rooms as soon as they went to uni so in the holidays one was sleeping in a home office and the other in a home gym. My mum was appalled and thought it was mean-spirited and not very supportive.
All that said, I have a 19 year old step child too! She didn't go to uni but works full time. We have a 3 bed terrace and I have 2 teenage sons too. They shared and the other room was for her. She was the one who said "you should give my room to (name of one brother) as he's getting older" I said "but we want you to have a space here, this is your home too" and she said "I know, but they can bunk in together just when I stay over". She stopped sleeping over much as she can drive and it's easier for her to get to work from her mum's house or she was doing things on the weekends with friends. No falling out, it's just how it's worked out. So I'd also say listen to your stepdaughter, what does she want and what does she need?
I was very mindful of the message it sent. I knew in practical terms she was with us less, when she was a child she was with us midweek and weekends so 3 nights a week. In her teens it fell to one, then dropped off through lockdown. But symbolically it was her space with her things in it. When she made the room change suggestion I made it very clear that she still had a space for her clothes and other belongings.
If we had 4 bedrooms, all 3 of our kids would have a room each. My older boy is starting uni this year and he's chosen to live at home for it but we've said he can redecorate the room over the summer so it feels a bit more grown up in there. He's been able to do his A level work in our home office downstairs but we're also getting an office base set up in his bedroom like he'd have had in halls.
Have a conversation with her about what she wants and needs. Don't lead her but she might surprise you.