def METHOD_NAME(sender, old_email=None, **kwargs):
    # All subs with the new email
    subs = Subscriber.objects.filter(email=sender.email, user__isnull=True)
    for sub in subs:
        # Find existing user subs on that newsletter
        exists = Subscriber.objects.filter(
            user=sender, newsletter=sub.newsletter
        ).exists()
        if exists:
            # Delete email sub in favor of existing user sub
            sub.delete()
        else:
            # Change email sub to user sub
            sub.email = None
            sub.name = ""
            sub.user = sender
            sub.save()