New York City Mayor Michael R. Bloomberg and city labor officials announced a tentative agreementto amend health benefits for more than 550,000 current and retired city employees, guaranteeing the city $400 million in savings over the next two fiscal years. The agreement imposes the $50 to $100 co-payments for about one-fifth of current and retired city employees, and eliminates coverage for preventive dental care at certain offices. For most other city employees, the plan would restrict certain hospital, ambulatory, and hemodialysis coverage to network providers and would implement several other administrative cost-saving measures.