Hi S.,
We put our son in his 'transition from crib to toddler bed' when he was around two. He did the crawling out thing, so we didn't have a choice.
When we did it, I had some reservations... what if he fell out?...would he get up and not stay in it?, those were my two biggest concerns. We let him help us with the transition part of the task. We had him lie on his "new" bed after we put it together and play on it, so he could get used to it. That night and for the first 2-3 nights, he got up, but we put him right back to bed telling him what we expected of him. It has worked like a charm, and he's never fallen out.
If there's room, you might want to set it up and let him get used to it that way. Keep both beds out for 2-3 days and tell him that he's getting to be a big boy and that by this Sunday night, he'll be sleeping in it from then on. Make it sound enticing so that he will want to sleep like a big boy.
Good luck!! ls