More job losses at the top ranks of Lehman Brothers are no surprise. Some executives are weary, just like their customers and most other Americans. Lehman Borthers is shaking up its top ranks amid deepening concerns about the firm’s financial health. Just before the market opened, New York-based Lehman said its president and the architect of its growth spurt in the middle of this decade, Joseph Gregory, will step down and be replaced by equity trading head Bart McDade. And the executive who has led Lehman’s aggressive defense against questions about its accounting and balance sheet, finance chief Erin Callan, will step aside in favor of co-accounting chief Ian Mowitt.