Which major brand identity overhaul did Delta Air Lines implement in 2007 that significantly repositioned its market perception?

The airline industry has undergone remarkable transformations in branding and customer experience strategies over decades. Delta Air Lines, one of the world's largest carriers, has a particularly interesting corporate history that reflects broader changes in transportation marketing. This trivia question explores a pivotal moment in Delta's brand identity evolution that helped position it as a premium carrier in the North American market.

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The 'Widget' logo redesign and introduction of the 'Keep Climbing' slogan
The 'Flying Colors' livery program featuring colorful tail designs for each aircraft
The merger-based 'New Horizons' campaign after acquiring Northwest Airlines
The 'SkyMiles Revolution' program that introduced premium cabin segmentation