Originally Posted by bhomburg

Icelandair only has two classes of service, Economy and Saga. Upgrade offers are always from Economy into Saga.
Economy standard and Economy Flex are only differentiated in that Flex tickets are refundable and have no change fees whereas Economy Standard tickets are non-refundable and incur change fees (in addition to the fare difference) if you wanted to change them. Seat and amenities are exactly the same otherwise.
If you intend to fly your itinerary as booked and have no plans to cancel or change your flights, book the standard fare and hope for reasonable upgrade offers. You will not gain anything by paying more for the flex fare if you do not need the additional flexibility those offer.