public function convertToInvoice()
    {
        $id = $this->ID;
        $this->ClassName = Invoice::class;
        $this->write();

        // Get our new Invoice
        $record = Invoice::get()->byID($id);
        $record->Ref = null;
        $record->Prefix = null;
        $record->StartDate = null;
        $record->EndDate = null;
        $record->write();

        return $record;
    }