Second baby nursery

The Doctor
I'm expecting my second child in July, and at that point my daughter will be 3.5. She has her own room right now, but we will need to make some changes when the new baby arrives. We have a large house (5 bedrooms), but 3 of the bedrooms are upstairs. Downstairs we have the master suite and my daughter's room. Her room is full of toys and covered in hand-painted murals. Right now, she doesn't sleep in her room; she sleeps in the bed with us. What I'm wondering is this.  Should I move her into an upstairs room and make her old room into a nursery? Going up and down stairs multiple times a day for diaper changes isn't going to be feasible. However, I don't really feel comfortable with my daughter going up and down the stairs alone. Our balcony isn't particularly safe (the railing isn't sturdy), and the hand rail on the stairs is too high for her to reach.
I could have them share a room, but then I worry that will cause jealousy.