Samples are taken from inside the cheek, maybe when head transplants are more common, but even then you'd need the donor to have committed the crime, left their

Samples are taken from inside the cheek, maybe when head transplants are more common, but even then you'd need the donor to have committed the crime, left their DNA via saliva, eyelash or such, then died, you receive the head transplant, then get collared for the crime. All without pointing out anywhere that it's a head transplant and could they use a different source of DNA sample.