About 20 million American workers were displaced in the 1980s, mostly from manufacturing jobs. Now, corporate restructuring and white-collar layoffs are adding highly trained workers and middle managers to the ranks of the displaced. Most laid-off workers will not get their old jobs back, and many are being forced to take new positions with less pay. The heated debate last year over the North American Free Trade Agreement (NAFTA) underscored concerns about the adequacy of U.S. programs for retraining displaced workers. In return for support on NAFTA, President Clinton promised additional assistance to laid-off workers. But now economists and other experts are questioning whether the country can afford a sweeping new plan -- and what the consequences may be if the current system is not overhauled.