We moved Nina into her big bed last month, and it's been a great success. She stayed in the same room, but we made a big deal out of it all. She helped us take the cot out, and we explained it was for baby Emily. Then when we set up her big bed, I put on her doona, telling her this is her special doona and pillow. And I took out all the 'baby' things, so made it up like a big girls room. She loves it so much that whenever someone comes over, she takes them into her room to show them her new bed, and we haven't had any problems with sleep either.
Good luck with it.