It seems that Nikon is finally ending the fiasco they had with the D600 once and for all. As you may recall, the D600 suffered a terminal illness which caused dust (or oil) spots to accumulate on its sensor. At first Nikon denied, than offered a free repair, and finally they were ordered to stop selling the camera altogether and brought on the D610 which solved the issue.
If you spent $1,800 on this camera this must have been a frustrating 2 years for you. Now Nikon is ending this marketing disaster once and for all and doing it quite well (although pretty late).
Procedure is not hard, but it seems limited to the persons who were in the initial lawsuit: