Current thinking

One line of thought is that 'paraphrenia' is basically a psychotic illness (e.g. schizophrenia). The differences (e.g. preservation of personality)
are simply due to the fact that the psychosis starts later, and so the person's personality has already formed (v.s Schizophrenia which often starts
early, and therefore disrupts personality development). I don't have a specific reference for this but might look for one at some point!