Hi,
This is actually the first post I have responded to. My daughter just turned 2 in Jan (1 week before my son was born). I had the same concerns about moving her out of the crib before the baby, etc. but decided it would be best for her to stay in the crib for as long as possible and worry about making the transition a month or so before we decided to move our son into the crib (we plan on keeping him in the pack-n-play in our room for 4-5 months).
Well when our son was 3 weeks old, our daughter learned to climb in and out of the crib so we didn't have a choice and had to move her to a "big girl bed". She LOVES her bed so that wasn't a hard transition, but it has been a struggle at nap time. She is a busy girl and a climber so naps have been a bit of a challenge since she can now climb in and out of her bed as she pleases.
If your child is not climbing out of the crib I would highly recommend keeping her in the crib for as long as you can! That being said, if you plan on putting the baby in the same crib that your 15 month old is currently using, I would recommend moving your child out of the crib at least 1 month before putting the baby in it so your 15 month old doesn't feel like she is being "kicked out" by the baby.
Hope that helps! Good luck!