Britney Spears, the pop star who brought sizzle to the schoolyard with glitter T-shirts and short shorts, strode onto a Milan runway last Tuesday evening in a $23,000 rainbow-spangled gown by Donatella Versace.

Spears, who turns 21 on Dec. 2, was flaunting her inner grown-up, turning to the makeover queen of couture for a quick fix. "She wanted something sophisticated and glamorous," Versace said.

It was the culmination of Spears' two-month intermission from work, ostensibly to relax but in reality to begin the process of refashioning herself for a new career. It will take more than one body-hugging dress and some nude chiffon to do the job.

Spears, who debuted as a wholesome bubblegum star with a penchant for sweetly flashing her belly button, is caught in a vicious conundrum of fame acquired young: The qualities that made her accessible and popular as a teen-age star may be precisely the ones choking her career as an adult, leaving her looking like an unseemly parody as she tries to become a grown-up recording artist.

Versace, who says she has known Spears for two years, said: "She understands that she has to change. We had a long discussion about it."

While Spears has sold 52 million albums worldwide in the last four years, sales have nose-dived, from 24 million for her first album, to 9 million for Britney, which was released last November, according to her manager - the decline shows her popularity is waning.

It is a pop-star crisis shared by a number of her peers, including Christina Aguilera, 'N Sync, the Backstreet Boys and a host of Britney clones, as they try to make the often hazardous shift from teen idol to adult superstar without alienating their loyal fans.

"The teen pop thing is mostly synthetic," said Jonny Podell, a co-founder of Evolution Talent Agency, which represents Spears and other young stars. "The majority don't get to the next level."