I would just throw her crib mattress on the floor and remove the crib. I'd put it on the floor in her room if you want her to sleep in there after the big kid bed is set up.
We had a problem with my toddler coming out of his room at night. He would be disoriented and looking for me in the middle of the night and I found him downstairs several times. We installed a lock on the outside of his door because it was scary to me to think about him roaming the house alone, esp. going down the steps all sleepy. (We tried a baby gate but he got through it no problem.) Now if he screams in the middle of the night, I hear him on the monitor and go get him and bring him to our bed.