If you want to keep the PD of the old case, you have to wait for the old I-140 to be approved, and also you have to get PERM and new I-140 approved, and then you may File 485 with the older case PD. And if your old company does not keep your I-140 anymore, you cannot use older PD.
Think about this, if you change your job, and start a new PERM, when the old and new I-140 are approved, EB3 PD could have almost reached your date 12/2004.