Delta changed thier uniform in 2001, and will debut this new one designed by Richard Tyler (thats a famous, good designer) in 2006. Song is already wearing uniforms designed exclusively by Kate Spade.
The costs are much the same as changing a livery- you have to do it eventually, so why not phase it in slowly. The perfect time to start is while they have a smaller workforce (Delta actually plans on calling back, its amazing). Five years for a uniform is a long time. Fast food restaurants change uniforms constantly, as do hotels and other customer service industries.
A new look is a major perception change for customers and employees alike. Like US330 said, the US Airways uniforms are dreadful. They are the most drab and generic airline uniforms I've seen. They look cheap- if you look at some peoples uniforms, Delta's curent one or American, they most likely cost the same but dont look cheap. Our uniform doesnt represent anything... smack on some wings, your a F/A. No stripes, no nothing. No colour so people might guess what airline we're representing. No way to distinguish between gate personel, flight attendants, Express employees, cart drivers, customer on his way to a funeral... There have only been slight changes to the uniform in twenty years (keep the uniform, we'll mail you the buttons with our new corporate idenitity-its the dawn of a new day!).
I could rant on and on about US Airways blatant disregard for its image and how it relates to its current position and inevitable downfall, but anyone can pull up my previous posts if they wish.
And oh yeah, marketing... I think Delta's fashion show on Morning Television was great advertising.
See here