Normally I trust FT over a random phone agent who may or may not be familiar with the policy. Have definitely been given inaccurate info on the phone before.
Given the divergent opinions here I contacted DL on Twitter and as has been posted upthread, no complimentary upgrades on this route.
Response from DL below.
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Thanks for your patience while I researched your concerns. JFK to KEF is considered Transatlantic. Complimentary Medallion Upgrades to Premium cabin are not offered unless you have a Global Certificate. If you space is not available to upgrade, you may be wait-listed for the upgrade. I hope this clears up some of your questions. Thank you for tweeting us today. *NW