Creating a single-payer health–care system in the U.S. would be a “major undertaking that would involve substantial changes” to medical coverage, according to a report issued by the Congressional Budget Office. The nonpartisan agency that evaluates the potential budgetary, economic and other effects of legislative proposals didn’t assess any specific bill or estimate costs. “The transition toward a single-payer system could be complicated, challenging and potentially disruptive,” the CBO said on Wednesday.