Only those holding tickets issued as Flex can "go show" in this manner.

Otherwise, you would have to pay the change fee associated with your ticket, and buy up to a Flex fare. This would, in some cases at least [cheapest available ticket bought well in advance of travel], cost more than what you initially paid for your ticket.

However, Light tickets issued in most markets (except for departures from Denmark, Spain, Finland, Norway, Netherlands, United Kingdom and Sweden) are not changeable; in such a case, you would have to buy a brand new ticket.