Several hundred Washington-based employes of the Postal Service, the government's largest agency, will be moved to other jobs, sent to field assignments or encouraged to retire under a long-awaited reorganization announced yesterday by Postmaster General Paul Carlin.

None of the Postal Service's 650,000 rank-and-file workers will be affected by the changes, which are limited to 2,800 employes currently assigned to headquarters, an Alexandria test center, training centers in Potomac and Rockville and the technical training center in Norman, Okla.

The cuts will be based on staffing needs of each facility and will affect a range of jobs.

Carlin said the reorganization, due to be completed in about a year, is designed to remove several layers of decision-making staff at headquarters and delegate more authority to the field.

Jobs that could be abolished range from assistant postmaster general to general manager and some clerical functions. None of them has been identified yet, officials said yesterday.

The changes are part of a shake-up Carlin announced in July to cut postal costs, then running at $1 million a day.

Carlin cut his own salary and that of top staffers, delayed raises due 700 top managers and grounded the service's executive jet at National Airport. Last month he said the service's financial position had improved, largely because of cutbacks in overtime. As a result, the service was $29 million under its budget for the month of August, officials said.

The headquarters reorganization will abolish 10 to 15 percent of currently filled jobs. Some employes will be transferred to field jobs -- with no cut in pay or grade -- placed in other federal agencies or offered early retirement.

Carlin has asked the Office of Personnel Management for authority to grant early retirement (on immediate but reduced annuities) to all affected headquarters employes with at least 25 years of service, or who have reached age 50 and have 20 years' service. Foster Grandparents Program

A White House luncheon is scheduled today to honor the 20th anniversary of the program that this year has matched 19,000 senior citizens with troubled, disabled or lonely children.

Under the program, a favorite project of Nancy Reagan, low-income men and women get a small fee for 20 hours of assistance each week for youngsters needing tutoring, advice or just a friend. ACTION, the national volunteer agency, runs the foster grandparent program. People

Federal Managers Association's Bun Bray will step down as executive director at the end of the year, but continue to manage the association's magazine. He has been with FMA since 1972. Before that he was longtime staff director of the House manpower subcommittee, developing and riding herd on legislation when Congress was improving federal pay and pensions.

Rep. Patricia Schroeder (D-Colo.) will speak at the Nov. 6 dinner of Federally Employed Women's Northern Virginia Chapter at the Rayburn House Offic Building. For reservations call 377-5691.

Jane McMichael has left the American Federation of Government Employees union to form a human resources consulting firm. She had been director of legislative and political affairs for the union.

Senior Executives Association's Jack Niles is now doing double-digit mileage training to prepare for next month's Marine Corps Marathon. He's living proof there is still life -- and energy -- after federal retirement.