Diet coke is updating its packaging by permanently switching to a design introduced on limitededition packaging last fall. The design, to begin appearing on diet coke products as of sept. 1, was created by san franciscobased agency turner duckworth. It features a section of the diet coke logo, cropped to feature the "d" and the "k," set against the brand&39s signature silver backdrop. The brand is simultaneously rolling out the latest extension of its &ldquostay extraordinary&rdquo campaign, with three new 15second television spots to begin airing on sept. 3, during the u.s.