Have you received a "Class UP" e-mail from Icelandair? You normally get one 10 days before your flight, and you can bid for a one class upgrade. As you have an EC ticket, you would be bidding on Saga class seat.
Per the T&C of 'Class UP", you have until 72 hrs prior to your flight to place a bid. Icelandair can accept a bid any time after you make it, but is under no obligation to accept any bid.
In your situation, perhaps they are waiting to see if EC passengers on your flight are going to bid for the upgrade, how much the bid is, and if they want to accept that bid and move those EC passengers to Saga.
Not all flights have "Class UP", but if yours does, I would think they are looking to sell the upgrades rather than just move people up from EC.