Millions of retirees have opted out of traditional Medicare over the past two decades, choosing to join a privatized, managed-care version of the program. But the choice might not be in their hands much longer.

The U.S. government has quietly launched a large-scale test of a new model for traditional fee-for-service Medicare that critics argue could transform it into another type of privatized managed care. Under the model, Medicare contracts with healthcare provider groups that receive a flat annual payment to provide care for enrollees in the traditional program. Some current fee-for-service Medicare enrollees are being placed in these so-called Direct Contracting Entities (DCEs) in 38 states where the pilot test is under way.

Learn more in my Reuters Money column.