Like creatures that shed their old skins once a year, the automobile companies used to burst forth each fall in fresh coats of new-model styles, options and optimism. Not this time.

Battered by continuing poor sales, foreign competition and high interest rates, the U.S. companies this fall are not merely changing their appearance, they are midway through a deeper, metamorphic change into something very different from the auto industry of the 1970s.

Detroit's Big Three -- General Motors Corp., Ford Motor Co. and Chrysler Corp. -- have been obliged to introduce a few carefully targeted new models this fall, concentrating their scarce capital where they think it will do the most good. A large part of Detroit's transition is the conversion of 1970s models to smaller, more fuel-efficient front-wheel-drive versions, and the huge costs of those changes are forcing the companies to bet their chips cautiously.

GM is making its expected move to dominate the midsized car market by introducing its "A" models by Chevrolet, Pontiac, Oldsmobile and Buick. There will also be new Chevrolet and Pontiac sports cars.

Ford is bringing out a five-door, hatchback Escort and a down-sized pickup truck. The new Escort model pushes Ford further into the smaller-car market, where the Escort is now its mainstay. The pickup is meant to maintain Ford's strength in the truck market, which is essential to the No. 2 automaker.

Chrysler will offer two luxury versions of its bread-and-butter "K" car, the Chrysler LeBaron and Dodge 400, and a subcompact pickup. American Motors Co.'s only new model will be a sports car from its foreign parent, Renault.

Among the leading importers, Nissan Motor Co. will introduce a new front-wheel-drive subcompact called the Stanza, the first of the company's U.S. exports that won't bear the name Datsun. Volkswagen will have a new compact to succeed the Dasher.

The new U.S. models are not all coming in a rush next month as in the past, however. The introductions, notably GM's "A" models, will be spread out into next spring, because of the expense and because of Detroit's new preoccupation with quality, auto executives say.

Last spring GM slowed the introduction of its "J" car compact line, the Chevrolet Cavalier, Pontiac J2000 and Cadillac Cimarron, to avoid painful recalls and to limit quality problems. "We have deliberately set very high standards" at the expense of production, said GM Vice Chairman Howard H. Kehrl at a briefing this summer.

The auto companies also must gear up for contract negotiations a year from now with the United Auto Workers, which will affect not only company and union relations and the quality improvement effort, but also the volume of Detroit car production that is shifted to other countries, industry officials say.

Chrysler and the UAW, forced into a new economic partnership by the government's rescue plan, have a profit-sharing arrangement that will figure in the UAW's bargaining with GM and Ford, industry analysts believe. GM and Ford want the kinds of wage concessions the UAW granted Chrysler, but they will be hard to get from the union as long as the two companies continue paying dividends to their shareholders.

Both GM and Ford have broadly hinted that an increasing share of their production of engines and components, and perhaps even assembled cars, will come from abroad if the companies don't get a satisfactory labor contract. That may happen anyway as the U.S. companies continue to reduce their overhead costs, according to industry analyst David Eisenberg of Sanford C. Bernstein & Co.

Buying abroad also gives Detroit better protection against guessing wrong on the kinds of models Americans will want, because it can limit investment in new capacity. GM is said to have put off its plans to produce a minicompact until after the UAW negotiations, according to industry sources, and is considering a joint venture with Japanese auto companies to produce such a car for the U.S. market.

The greatest imponderable is the direction of consumer sentiment. A combination of high interest rates and inflated new-car prices have undercut demand, forcing the companies to pump up sales through payment of costly rebates. The tradeoff was evident in the first three months of this year, when rebates of up to $1,000 per car pushed sales to an annual rate exceeding 10 million units (including imports). Ford's and Chrysler's losses totaled $738 million. GM had a $190 million profit. When the rebates were dropped, sales dropped too, to an annual rate of 8 million.

A 12-million-car sales year would make the industry healthy again for a few years, at least, and the companies continue to believe recovery is just beyond the horizon. But it still isn't clear whether consumers have accepted the "sticker shock" of new-model prices and high financing charges. The cost of car ownership is now more than $225 a month, analysts say.

The overdue sales boom isn't expected until mid-1982, when the industry is counting on the effects of a federal tax cut. Eisenberg projects a $1 billion profit for GM this year but foresees losses of $534 million for Ford and $376 million for Chrysler.

Those figures wouldn't change the relative competitive positions of the companies. GM remains in the strongest position by far, with the biggest financial reserves and the largest array of new, fuel-efficient cars coming out in the next few years. Although GM is having trouble selling Chevrolets and Pontiacs, losing market shares to Ford and Chrysler, it should be in a dominant position relative to its U.S. rivals, assuming the auto market picks up in 1982 and 1983.

This winter, GM will introduce the "A" line of front-wheel-drive "family" cars, the Chevrolet Celebrity, Pontiac 6000, Buick Century and Oldsmobile Cierra. No other competitors will be able to match the range of cars and trucks that GM will offer in the mid-1980s.

Having defied critics' predictions of an early demise for Chrysler, its chairman, Lee A. Iacocca, could say "I told you so" this summer, thanks to the success of the hot-selling compact K-cars. Chrysler's compact car sales for the first half of 1981 were 129 percent greater than in the same period a year ago.

But even these gains won't make Chrysler profitable for the year. It can lose $350 million and still be in compliance with the Treasury Department's financial plan, says Iacocca, but he acknowledges that the company will have to find a merger partner in the mid-1980s. By itself, Chrysler cannot repay the $1.2 billion it owes the government while making good on debts to retirees and creditors, industry analysts say.

Chrysler's problem is that it depends almost entirely on its line of Omni and Horizon subcompacts and the very successful compact K-cars. Chrysler doesn't have the money to produce a new subcompact to replace the aging Omni and Horizon, and although the K-car line will be expanded by the introduction of a luxury version this fall and a convertible next spring, the K cars have to compete in the toughest, most crowded part of the automobile market.

Ford's future is the hardest to foretell.

Unlike Chrysler, Ford publicly holds onto the dream of regaining its place as a full-fledged competitor of GM with a complete line of cars and trucks of nearly all sizes. The continuing slump pushes that hope farther from realization.

David Healy, auto industry analyst for the Wall Street firm of Drexel Burnham Lambert Inc., noted that Ford has cut its North American production schedule for the rest of this year by about 10 percent, leaving Ford with a small increase in production over 1980. His estimate of Ford's 1981 loss, more pessimistic than Eisenberg's, is over $800 million (assuming lower production overseas).

Ford's finances continue to erode. Its working capital dropped to $306 million on June 30, from $1.7 billion a year earlier and $3.3 billion in June 1979. This makes it harder for Ford to pay for the modernization of plants and development of new products it must have to keep up with GM in the mid-1980s. But Ford still has a large line of untapped credit with its banks, and can probably get that money as long as the bankers share Ford's optimism about a revival in auto sales.

On the plus side, Ford continues to cut its overhead costs heavily, Healy and Eisenberg agree, putting it in a position to make big profits if auto sales do make a strong recovery.

In the industry's view, the recovery won't occur until interest rates fall. Wendell Miller, New York State auto dealer and president of the National Automobile Dealers Association, says the group is still urging its members to be cautious, anticipating that interest rates will remain high for the next three months at least.

"Long range, I'm very positive about the auto business. The survivors will be successful," Miller said. "In the short range, it's still a disaster."