When Calvin Klein designer Francisco Costa won the prestigious Council of Fashion Designers of America award for womenswear designer of the year earlier this month, it marked an interesting moment in fashion: It was the first time that a designer won the prize for a collection that bore the name of its still living founder.

There’s nothing new in designers working for labels named after somebody else. Successful fashion houses have always outlasted their founders. Yves Saint Laurent launched his first collection at Christian Dior in 1957, and the house of Lanvin has had at least 9 couturiers since 1946. But in the old days, new talent only stepped in after the founder died. Now, creating a label in your name is no longer a lifetime job guarantee. The question is, does anyone care that Calvin Klein is no longer “Calvin Klein”?

“No matter how brilliant a designer is, there is always an expiration date,” the president of the fashion syndicate French Federation of Couture, Didier Grumbach, said. Designers usually manage to keep up with fashion for one generation, he said. (A notable exception is Karl Lagerfeld, whose career has spanned from flower children to generation Y without signs of slowing.) When the buzz around a label has died down and reviews are more polite than raving, a designer can find him or herself pushed out by his or her label’s corporate management. While the old houses were co-owned by the founding designer and a business partner (who often did double-duty as the couturier’s therapist and cheerleader), most major fashion labels today are owned by large luxury conglomerates that hold more control of the brand than its founder does. Thus an anxious company management can replace the founding designer with a younger talent, if they feel that such a move would increase profits.

So what happens on the retail floor when a designer leaves a label? Do sales figures change at all? Not necessarily, according to Jeffrey Kalinsky, proprietor of the high-end boutique Jeffrey New York. While a small percentage of the population reacts quickly to fashion news, most shoppers only pay attention to the look of the clothes. If a new designer successfully changes the style of the clothes, he or she will attract a new clientele. “A label’s sales can stay the same when its designer changes, but then the customer also changes,” Mr. Kalinsky said. He added that there are two kinds of customers. “Type A comes into the store because they need clothes. They’re open-minded shoppers who are not terribly concerned with what label they buy, as long as it fits and works well with their wardrobe. Type B is a highly knowledgeable fashion fan and very much devoted to his or her favorite designers.” While the latter shopper feels a personal loss when a label drops her pet couturier, the same event is hard ly a blip on the retail radar for other shoppers.

Clearly, there are more type As than Bs at Macys. “If you ask a mainstream shopper who designs for Dior they would probably say ‘Christian Dior,'” the creative director for the trend forecasting firm Doneger Creative Services, David Wolfe, said. According to Mr. Wolfe, the general public isn’t concerned with designer musical chairs. “There’s a credibility gap between what the media considers successful and what is really successful commercially,” he said, pointing out that despite Mr. Costa’s critical success, Calvin Klein’s sales have not increased since he took over the women swear division in 2003.

In general, if a label already is selling well and has a firmly established image, the transition of its head designer will be relatively smooth. A case in point is the recent designer switch at Gucci. When Tom Ford designed for Gucci, he was one of fashion’s superstars, and his abrupt split from the label in 2004 was the biggest fashion news of the year. Yet Gucci’s sales have continued to rise, even though the name of its new creative director, Frida Giannini, is hardly familiar to anyone outside hardcore fashion circles. Even the fickle, “Type B” shopper can remain loyal to a label that changes hands, if the move is done well. “I still have a soft spot for Gucci,” Sarah Issleib, a stylish New York public relations director, said. “The mysteriousness and allure Tom Ford created is still there somehow, but I also like Frida’s new, fresh approach.” Ms. Issleib added that usually, her fashion choices are based on loyalties to a favorite designer. “I’m definitely less interested in Chloe now that Phoebe Philo isn’t there.”

Despite the fact that most shoppers don’t keep up with the merry-go-round of designers, those changes can still hurt a label’s business. If a fashion house relies on very loyal shoppers, the wrong choice is noticed quickly on a retail level. “The problem is that the people who make the decisions about hiring designers are businessmen who don’t understand fashion,” an agent for Givenchy’s haute couture business in America, Barbara de Portago, said. Indeed, Givenchy has had its share of ill-fated designer appointments. In the past nine years, the label has hired and fired three designers (Alexander McQueen, Julien McDonald, and Riccardo Tisci) with very disparate styles. The switches have confused critics and pushed the business into a downward spiral.

Even though the patterns of that rare, clued-in, Type B shopper don’t reflect the mass market, it’s likely that they do set the direction for future trends. At the same time as the B types are quick to reject a label that no longer employs their favorite designer, they are also the consumers that are most willing to try new things. Ms. Issleib, the public relations director, said she tends to be intrigued by a previously shunned label if it has switched designers. “I think it’s great when a label isn’t stuck in a look, but dares to change with its times. It takes courage to take a chance with a younger talent,” she said. “I want to support that.”

Support of that kind can lead to mainstream stardom. That was how Tom Ford’s rise at Gucci began in 1995. For Mr. Costa, with his growing fan base and industry acclaim, perhaps it’s just a matter of time before the sales figures start to reflect his new status. He’s not yet Calvin Klein, but soon, being Francisco Costa might be just as impressive.