SYDNEY — Australia has its work cut out keeping aging Collins-class diesel-electric submarines in service as it transitions to nuclear-powered submarines, firstly to second-hand Virginia-class boats from the United States, and then to SSN-AUKUS submarines designed and built in conjunction with the U.K. Asked about the risks for the Collins-class life extension and the SSN programs, Rear Adm. Stephen Hughes, head of naval capability for the Royal Australian Navy (RAN), said he holds a “half-