groundhogg/contact/owner_changed

do_action( 'groundhogg/contact/owner_changed', $owner, $contact, $prev_owner );

add_action( 'groundhogg/contact/owner_changed', 'my_extension_notify_new_owner', 10, 3 );

/**
 * Email the new owner when a contact is reassigned.
 *
 * @param \WP_User             $owner
 * @param \Groundhogg\Contact $contact
 * @param \WP_User             $prev_owner
 *
 * @return void
 */
function my_extension_notify_new_owner( $owner, $contact, $prev_owner ) {
    \Groundhogg\gh_mail(
        $owner->user_email,
        'A contact has been assigned to you',
        sprintf(
            '%s (%s) has been transferred to you from %s.',
            $contact->get_full_name(),
            $contact->get_email(),
            $prev_owner->display_name
        )
    );
}