"The Only Constant is Change," shouted the New Zealand Herald's headline welcoming the 21st century today. It wasn't referring to the America's Cup--but it could have been.

When the six-boat, 10-day semifinal round of the Louis Vuitton Cup challenger series opens Sunday with three of the boats flying U.S. flags, much will be shrouded in the sort of mysterious fog that all but swallowed celebratory fireworks over Auckland on Friday night.

"We won't really know what anyone has until three or four days into the week," said Moose McClintock, jib trimmer on top-rated U.S. entry AmericaOne. "Everyone's changed their boats. We'll have to wait and see what they've got."

With only two boats advancing to the challenger finals Jan. 25, the uncertainty has AmericaOne skipper Paul Cayard fretting.

"I haven't slept a lot," he said wearily as his twin race boats were readied for a last day of testing today. "This is the scariest part of the whole thing--racing against a different boat every day for 10 days with all the variability of the weather.

"It's scary for strong teams like ours. Anything can happen. You might draw Stars & Stripes on a heavy-air day when they're good, the French in light air when they're tough. In the last round, there were crapshoots when the wind shifted 180 degrees and anyone could win, though hopefully that was an extreme we won't see again.

"I'd like to race each of these guys three out of five. Then we'd either win or lose because we deserved it. But racing a new guy every day, the permutations are huge. . . . "

"It's six good boats and no days off," said mainsail trimmer Terry Hutchinson of Annapolis. "You get a bad start, it's going to be hard to get around 'em. If we fumble a few times early, which is easy to do, we'll have a hard time moving on."

For all its worries, AmericaOne remains a co-favorite to advance to the challenger finals alongside Italian entry Prada. On most handicappers' cards, the front-runners are followed in order by Japanese entry Nippon; San Francisco's America True with the first female Cup syndicate chief, Dawn Riley; Dennis Conner's Stars & Stripes and the French Le Defi Francais.

But anything can happen.

Most mystery surrounds the top three, all two-boat programs bringing essentially new boats to the semifinals. Nippon and Prada introduced new boats to the last round robin in November but are going back to their first boats, modified for more speed.

What exactly did they change? "We know Nippon got much faster when it brought its new boat out for the last round robin," McClintock said. "Now they've modified the old boat and it looks faster than the new one."

Meantime, Prada returned to its older boat after losing twice in Round Robin 3. The Italians had lost only one race in the first two round robins and may have given up a step with the newer boat.

Only AmericaOne saved its new boat for the semifinals. USA 61, the last boat built for this Cup, arrived in October but wasn't ready for November racing. Cayard & Co. have been testing the crisp newcomer against their older boat for three weeks. How is it going?

"Pretty nice," said Hutchinson, with a satisfied chuckle. "It's stronger in certain conditions than we expected." He wouldn't volunteer which, but logic dictates the newer boat would have been designed to go best in high summer, when winds are generally lighter.

Hutchinson chuckled again and nodded. "It's really slippery and easy to sail in light stuff," he said. "That's a big comfort. The nice thing is, it's better than we expected in heavy air."

As skippers and crews sweat and stewed, Auckland took on a festive air over the New Year's weekend with throngs crowding the Viaduct Harbor where the boats tie up. The racers have been joined by a fleet of gleaming yachts from around the world to celebrate sailing's premier event--J-boats like the restored Velsheda, the glittering schooner Shenandoah fresh from the Mediterranean and scores of other megayachts arrayed along the quay for Aucklanders to ogle.

On Sunday, attention shifts from the glorious gin palaces of old to the stripped-out, towering, fragile vessels battling the Hauraki Gulf's unpredictable winds and seas for the right to sail for yachting's crown jewel against Team New Zealand in February.

The lineup:

Prada--Italy's entry had the best record in three round robins with only three losses. Skipper deAngelis has proven unflappable and a clever boathandler in close quarters during the pre-starts. The Italians have a budget in excess of $50 million, two very fast boats, the top Cup designer in Doug Peterson and a superb tactician in Brazilian Torben Grael. But, said Hutchinson: "They sailed very well when they got here and they still do, but they haven't improved the way the rest of us have."

AmericaOne--Cayard comes to his fifth Cup fresh from a commanding win in his first Whitbread 'Round-the-World Race in 1998. Budget constraints hampered A-One early, but the money has arrived. The new boat, USA 61, looks fresh and fast and Cayard has the strongest crew in the regatta, with Olympic silver medalist John Kostecki, J-24 world champion Hutchinson and match-race world champion Gavin Brady.

Nippon--Australian-born skipper Peter Gilmour barks orders in English and the crew answers in Japanese, but they seem to have worked it out. After a rocky first two rounds aboard their first boat Asura, second boat Idaten proved quick; now Asura is back, purportedly faster still. Gilmour is the most aggressive starter in the fleet, which can provide him commanding leads early or big deficits if he commits a foul and must do a penalty circle. He caused a stir last week when he went practice racing for four hours against Team New Zealand, the Cup defender, a violation of unspoken no-fraternization rules that put fellow challengers Conner and Cayard in a fury.

America True--Skipper Riley's San Francisco-based effort was on few lists to go this far when the regatta began, but she has spent her small, $20 million budget wisely, come up with a fast boat and hired a good skipper in New Zealander John Cutler, a veteran Cup figure, and a first-class tactician in Californian Dee Smith. With a one-boat campaign, she lacks the ability to test upgrades in keels, rudders, sails and rigging the way the two-boat campaigns can, but True has proven quick and well-sailed in all conditions, and the first mixed crew, with three women including Riley aboard, wins friends at every turn.

Stars & Stripes--Conner, the world's most experienced Cup sailor, still runs the show but is rarely on the boat, having turned skippering over to perennial J-24 world champion Ken Read. The crew consists of grizzled veterans who know the game. So far, with a tiny budget of $10 million or so, Conner has managed to slay big dragons.

Le Defi Francais--The French benefited from largess by AmericaOne and America True, which lost or forfeited races in the final round robin to help get Le Defi to the semifinals and keep $40 million Young America out. Can the French survive? Not likely. But they could score consequential wins in light airs in their radically narrow, bright orange boat.