What happens if I want to book a more expensive cruise than the voucher value? Or a less expensive cruise?

If the future booking is of higher value than the original one, you will have to pay the difference. If the future booking is of less value than the original one, you will have the opportunity to add-on packages to your future cruise (beverage packages, excursions, etc…) and leverage the voucher value for such usage up until it is fully spent.