L.M.
I switched my son to a big boy bed when he was a little over 2 1/2 (technically 31 months). Like you, I was expecting baby #2 at the time, and did not want my son to have to deal with the upheaval of having a new sibling as well as moving into a big bed all at once. The transition was extremely easy and he was very excited to have a big boy bed (we made a big deal out of it and let him choose the new sheets). By the time the baby came a couple months later, he had totally forgotten that he ever slept in the crib and considered it the baby's. My son never got out of his new bed, and had no change at all in his sleep pattern. I was glad we switched him when we did.