Do you have a date for when you see FI 615 operating as a 76W?
Icelandair announced they are getting 2 767s next year, but their parent company, Icelandair Group, does have 767s in their lease fleet, so perhaps they have decided to expedite this.
They could certainly make good use of the additional capacity...everyone seems to want to go to Iceland, or TATL and make use of the Iceland stopover feature by flying Icelandair.