The Postal Service is making the announcement Wednesday, more than six months before the switch, to give residential and business customers time to plan and adjust, the statement said.
"The American public understands the financial challenges of the Postal Service and supports these steps as a responsible and reasonable approach to improving our financial situation," Donahoe said. "The Postal Service has a responsibility to take the steps necessary to return to long-term financial stability and ensure the continued affordability of the U.S. Mail."
He said the change would mean a combination of employee reassignment and attrition and is expected to achieve cost savings of approximately $2 billion annually when fully implemented.
The agency in November reported an annual loss of a record $15.9 billion for the last budget year and forecast more red ink in 2013, capping a tumultuous year in which it was forced to default on billions in retiree health benefit prepayments to avert bankruptcy.
The financial losses for the fiscal year ending Sept. 30 were more than triple the $5.1 billion loss in the previous year. Having reached its borrowing limit, the mail agency is operating with little cash on hand.
The agency's biggest problem - and the majority of the red ink in 2012 - was not due to reduced mail flow but rather to mounting mandatory costs for future retiree health benefits, which made up $11.1 billion of the losses. Without that and other related labor expenses, the mail agency sustained an operating loss of $2.4 billion, lower than the previous year.
The health payments are a requirement imposed by Congress in 2006 that the post office set aside $55 billion in an account to cover future medical costs for retirees. The idea was to put $5.5 billion a year into the account for 10 years. That's $5.5 billion the post office doesn't have.