I had the same issue. My son was 19 1/2 months when our little girl was born, and I didnt' want to go out and buy another crib that we'd just use for a few months. So, a few months (maybe 2) before my daughter was born, we transitioned my son into a pack n play. He had taken some naps in it before, and we have used it when we travel. I started with his daytime naps, and put the pack n play in his room with the crib still there. at night, we stuck to the crib. We did this for a few days, and then put him in the pack n play at night, and it went so smoothly, we were amazed. he never asked to be put in the crib, and we didn't offer the option anyway. After a few days of this going so well, we took the crib apart and put it in the new baby's room. I;m not sure if it helped, but we didn't reassemble it til it came close to the due date.
So, it;s worked out well for us. Granted the pack n play isn't as roomy as a crib, and my son doesn't have much wiggle room, it's never been an issue, except the grandma feels sorry for him-he is totally content in there and plays in there when he wakes in the morning.
one thing, is the cushion/padding is so thin, it's not as comfy, so i found a nice plush pack n play sheet cover by eddie bauer, and he seems happy. He's been in there for about 5 months now!