Banks rolling over some of their Greek debt into new instruments may have to take impairment charges, Moody's Investors Service said Tuesday, in another setback for efforts to involve private bondholders in a new international bail-out. Rival agency Standard & Poor's Corp. on Monday rocked plans to involve the private sector in giving Greece more time to work out its fiscal problems by saying a proposal being promoted by French banks would likely put the country in "selective default."