Former painter here. My recommendation would be to prime the entire car to seal off the old paint to prevent any reactions between the old and new paint, after a thorough sanding of the old substrate of course. If you dont want to go that far, then at least put a coat of sealer down before your color coats to help prevent problems. If you shoot directly over top of your old paint you risk the old interacting with the new and causing bad things in your new paint job.