My DD swapped to a bed at about 2 yrs old. I put her bed up and left the cot in her room as well incase she wanted to get back in it. But she never did, loves her big bed
If your going to use the cot or the baby (if u can) i would move to a big bed atleast a lil while before baby is due to comvert from bassinette to cot otherwise your DD might feel your taking it from her and giving to the baby.
Just a thought, not neccessarily what will happen.