M.S.
I moved my son to a big-boy bed when he was 2 to prepare the crib for my daughter who arrived 3 months later. We did not have a second crib. Also, my son didn't sleep though the night in the crib. He slept so much better in the big boy bed, we wished we had done it earlier! Plus, it gave him a sense of growing up to not be in the baby bed anymore. This helped eliminate the displacement feeling when baby sister arrived. He was excited to show her her bed and room when we brought her home. We never had problems with him crawling out of bed (and we might well be in the minority there). He did cry out for us, just as when he was in the crib. We'd go in to settle him and he'd go back to sleep. A very tight bedtime routine helped us: once you get into bed, you stay there until morning time. He still doesn't get out of bed at night and has been potty trained since 2.5 years. He's four now, and is asking when his baby sister can sleep in a big girl bed in his room!
Our bed for him has a head and foot board and we pushed one long side up to the wall and stuffed the space with extra blankets. We also used an extra long bed rail from One Step Ahead. There was a small gap that allowed him to get in and out of the bed near the foot. Only once did he ever fall out. He fell halfway out, head first and we have no idea how that happened.
Use the time you have now to prepare him. You won't have much time to do so later, and he might get jealous of new baby.
Good luck!