T.S.
Hi E.,
I switched my son to a big-boy bed (a little tykes race car bed) when he was 21 months old. We had another child due in 3 months and we needed the crib for her.
We had a party to commemorate the big step. We decorated the bed, had a cake, and invited over some family members. We also brought him to the store to have him pick out the sheets/blanket he wanted for his new bed.
Giving your crib up for another child can be stressful, so we took all the attention off the crib/baby and put it onto the new bed and our "big-boy". It worked really well. He never asked to go back into the crib, even when the baby arrived.
I hope this helps.
T.