D.,
Of course every child is different, but here's our story. My son was a great sleeper in his crib. He never, ever tried to climb out. We were given a toddler bed and just put it in the garage. Eventually, my son saw his other friends with their toddler beds and thought he wanted a "big boy bed" too. So in June, at 38 months, we went for it. Let him pick out his own sheets, made a big deal about putting it in his room, took down crib, said "goodbye crib"...
He wouldn't stay in the bed. After sleeping next to him on the floor for almost 2 months in an effort to keep him in the bed, we quit, put the crib back up and if he got out of bed more than once, he slept in the crib!
About 2 weeks ago, we noticed that we hadn't had to put him in the crib in about a month so we asked him if he was ready to put the crib away. He agreed and so far so good, except if he gets up in the middle of the night. Then he wants one of us in his room and will keep getting out of bed until one of us is sleeping there again. He had never gotten up in the middle of the night until recently and it is because he is wearing big boy underwear to bed and sometimes having accidents. I'm assuming this too is a phase and he will eventually stay dry and I'll be back in my bed.
So, if yours isn't trying to get out of the crib...just either buy or borrow a crib for the new baby. You could easily have your first in the crib for another year or so...
Plus it is much easier to know that your older is staying in bed and not wandering around, or pitching toys at your sleeping infant.
Cheers,
C.