The greater dispatch reliability of the -8's over the 400's is an asset.

The -8 warranties cover repair of major components for first four years and there is a heavy maintenance “honeymoon” that Boeing pays. That cost savings is an asset.

The 747-8 has more than a 15% fuel efficiency advantage over the 747-400, 16 percent more revenue cargo volume than the 747-400 Freighter, four additional main-deck pallets and three additional lower-hold pallets, and an additional 22 tons (20 tones) of payload.

Based on the poor dispatch performance of a couple of our older maintenance plagued 400's, I think "replacing" them with -8's would be a wise thing to do.