Imagine if the baffled survivors on Lost suddenly packed up and moved to Manhattan, where their existence was threatened not by the Others but by gypsy cabs. Or the bad-boy hedonists of Las Vegas all got transferred to Des Moines to work in a Denny’s. Suppose the taut life-and-death drama of Grey’s Anatomy was set next year in a shopping mall instead of a hospital.

OK, now you’re getting an idea of the challenge facing teen dramas Veronica Mars and The O.C. this season, now that their characters have graduated from high school and set off into the world. Except their producers don’t seem to see it as a challenge at all.

’’Leaving high school is chump change compared to killing off a major character, which we did at the end of last season,’’ says Josh Schwartz, executive producer of The O.C. Agrees Veronica Mars executive producer Rob Thomas: ``Shows fail not because your characters go to college, but because you take so long getting there. If you’ve had your characters in high school for six or seven years, you’re probably on your last legs anyway.’’

Plenty of prime-time shows are returning this season with significant changes in their landscape. On CSI: Miami, David Caruso will be chasing criminals overseas. Prison Break should probably be retitled, Prison Broken: The inmates who spent last season plotting their escape are out and hunting for buried treasure. And both of the Gilmore Girls are coming back without their menfolk.

But nothing matches having your cast graduate from high school, leaping from the highly structured environment of adolescence into a world where anything can happen. That’s why kids, from the sainted Ricky Nelson and Wally Cleaver to the delinquent morons of Welcome Back, Kotter stayed in high school until they were nearly old enough for Social Security. And producers bold enough to make the big leap did so with eyes closed and hands clasped in fervent prayer.

’’I do remember a sense of trepidation the season we had everybody graduate in Beverly Hills, 90210,’’ says Darren Star, a producer who successfully made the conversion and four years later did it again when his characters finished college. ’It was like, `What are we going to do next?’ But in the end, it really helped . . . Especially for a long-running show like 90210, it opens up new new stages to play on.’’

Neither The O.C., entering its fourth season on Fox, nor Veronica Mars, on the new CW network after two years on UPN, have anything like the longevity of 90210. But producers of both shows say they’re ready to shake things up.

’’We may have even have gone a year too long in high school,’’ says Schwartz, whose O.C. has revolved around a kid from the wrong side of the tracks suddenly thrust into an ostentatiously upscale oceanside community.

``Even though the characters were high school students, our show was never a high school show. The first seven episodes, when we started building our audience, took place in the summer, so no one was even in school. Our whole thing was kids acting as adults ahead of their time, while the adults acted immaturely.

``Last year we made it more a high school show, with plots about mean administrators and drama-club productions, and that just made it seem like an average teen drama. It was not a great fit.’’

This season, The O.C.’s four teenagers — well, three, now that tormented bad-boy addict Marissa was killed off in the season finale last spring after the writers became bored with her endless tragedies — will head in vastly different directions. Marissa’s on-again-off-again boyfriend Ryan will abandon college out of guilt and return to his working-class roots. Social fluffhead Summer, on the other hand, will go to Brown and become a nut-case ecoterrorist. Her boyfriend Seth, left behind, goes to work in a comic-book shop and reverts to his original lonely-nerd status.

’’It’s exciting to be working with all these new stories,’’ says Schwartz. ``In some ways, it’s like a whole new show.’’

Veronica Mars producer Thomas says his show — a gritty, class-conscious version of Nancy Drew — is also not really about being a teenager. ``I think leaving high school would have been scary if we were a coming-of-age drama. Freaks and Geeks, for instance — that was their stock in trade. It was really about being 15 or 16. Our show is a straight-up mystery. That’s a different animal.’’

Saying goodbye to high school, Thomas says, will enable him to toss aside a frequent Veronica Mars theme that had begun to bore him: rich and popular vs. poor and outcast.

’’When he was producing Buffy The Vampire Slayer, Joss Whedon used to talk about how much he liked the metaphor of high school as a monster movie — high school being all about demons,’’ Thomas says. ``We modified that to high school as a noir universe. We can play with that in college. Veronica is going through sorority rush, but she’ll soon say, `I’m not one of these people, I don’t like them, I don’t want to be them.’

``But there’s going to be much less stuff about who’s got the cooler car and that sort of thing. I’m really tired of playing Veronica against the rich bad boys.’’

Instead, Veronica Mars will deal with political and cultural themes that seemed too broad for high school. Her fictional hometown campus, Hearst College (where one of the trustees will be played by the real life heiress Patricia Hearst) will be terrorized by a serial rapist.

’’That will enable us to bring in a lot of the political issues surrounding race and gender that you’re seeing in the lacrosse-team rape case at Duke,’’ says Thomas. ``I wouldn’t buy that at all on a high-school campus, but it’ll work in college.’’

Thomas was so anxious to break Veronica Mars out of what he felt was an increasingly suffocating box that he nearly sent her to college out of town at Stanford, hundreds of miles from the show’s Southern California. The debate in his mind played out in the show, with Veronica trying for — but ultimately losing — a Stanford scholarship.