I moved my son over to a big boy bed when he was 19ish months I believe... to make room for baby #2 as well. It went great. I was apprehensive at first but giving him his own room with a few special new toys just for the "big boy" made him never look back. He started calling the old room the "baby room" that very day.
Sure he falls out/climbs out occasionally, but as long as you are consistent with discipline and not let them roam around the room but put them back right away and say they can only get out when mommy/daddy lets them out, all will be well.
In the end I think it is better to do now than later. Why have two cribs when you will have to move him in a few months anyway? Most kids learn how to climb out of the crib anyway and even if they don't, I think it goes a long way in showing the "big boy/girl" their position in the family so that when the new baby DOES come, they are "big" helpers in that area too and don't still think they are "the baby."