K.U.
My kids are a little under two years apart, but my daughter was 18 months when we put her in a big girl bed. She transitioned fine. I didn't make too big deal about it, but made it exciting when I presented the option of sleeping in the big bed. Some kids are more willing to change...others not as much.
She had a couple months in the bed before her new brother got in to the crib. I think it is important that the child doesn't think that they are moving to a new bed...just because the baby is taking theirs. My son slept in a bassinet for a while, just because that was easier for me. Then I put him in the crib when night feedings weren't quite so often.