K.W.
Hi I am in a similar situation but luckily my second child is older. I would definetly leave the 15 month old in the crib and get another one for the new baby. My peditrician always said leave them in the crib as long as you can because they will never sleep as well once they are out. My 2 1/2 year old was moved to a bottom bunk bed Memorial day weekend and she has done okay but even now as she is more aware of the new baby on the way she is regressing a bit...and wants to be babied. I think it will work out fine for her but for you and the young age I wouldn't fight it. I think it will be better for you and the 15 month old in long run to keep the crib. Maybe you can borrow one or get a used one for the new baby.
Good Luck!