Delta Air Lines announced today, that they will begin retiring their 16 747-451s that were integrated into the fleet when Delta merged with Northwest Airlines. Four 747-451s will be retired by the end of this year. It is not known when, or how fast, the remaining 12 will be retired. Of the 16 747s, six are leased.
Delta presently has one NRT-HNL 747-451 flight, DAL578/579, and appears not to be affected by this move so far, as the airline will downguage other 747-451 flights from Japan to the US mainland.
Further details in this FrequentBusinessTraveler.com article.