When a deputy sheriff came to his door with a court summons, George Kneifel, a retiree in Union Mills, Ind., was mystified. His former employer was suing him. The employer, beverage-can maker Rexam Inc., had agreed in labor contracts to provide retirees with health-care coverage. But now the company was asking a federal judge to rule that it could reduce or eliminate the benefit. Many companies have already cut back company-paid health-care coverage for retirees from their salaried staffs. But until recently, employers generally were barred from touching unionized retirees' benefits because they are spelled out in labor contracts. Now,...