DETROIT — After months of waiting for the economy to improve, General Motors Corporation was expected to announce a major cost-cutting program yesterday that would slash thousands of jobs from its payroll and mark the end of an era for the world's largest automaker.Analysts said GM could cut as many as 25,000 white-collar jobs through layoffs or sweetened incentive packages, and close as many as seven North American assembly plants. Analysts said the moves are aimed at shrinking the giant automaker and improving its chances of being profitable in a more competitive market. The restructuring, which will cost GM billions of dollars, signals a fundamental shift in the company's method of operations, they said. Industry sources say GM may elect to close all or part of assembly plants in Arlington, Texas; Doraville, Ga.; Lake Orion, Mich.; Oklahoma City, Okla.; Oshawa, Ontario; and Willow Run, Mich. With GM's North American operations expected to lose $7 billion this year, analysts say GM's board members have abandoned hopes of recapturing the automaker's one-time hold on 45 percent of the United States market. GM stands to lose about $400 million in this fourth quarter, on top of $2.2 billion in losses so far this year. Much of that can be attributed to the industry downturn that has rocked Detroit and forced all three US automakers - GM, Ford Motor Company, and Chrysler Corporation - to look for ways to cut costs.