The Flex fare has this clause for all European routes, even those operated by 737s.
LY usually opens seating in an higher class of service for economy pax only when the Y class is forecasted to be oversold and only when there were no business class tickets purchased or when the plane has two business class sections so that one has only pax with true business class tickets and the other has economy class (only 777s/747s has this configuration).
Thus, this perk offered by flex tickets is not available for most flights.