Andrew Young, the Georgia congressman and Carter adviser who will take over as U.N. ambassador, plans to make a clean sweep of the top staff at the U.S. mission to the United Nations.

Sources at the United Nations and in Washington said he plans to fill the slots with persons who have foreign policy experience but who are now outside the State Department.

Young has already told the four resident ambassadors, all of them career professionals, that they will be replaced by his own people, according to a number of U.S. mission staff members.

In addition, several members of Young's congressional staff will be brought here fill posts down to the level of political officers dealing with African and Latin American affairs.

The offices of Young and James Leonard, who reportedly will become Young's chief deputy here, refused to comment today on the prospective changes.

"Like many congressman," said a Capitol Hill source in Washington who knows of the changes, "Young had deep distrust of the U.S. foreign service pros." He has been given a free hand by Carter to pick not only his own staff here at the mission, but also the upper echelon of the State Department's Bureau of International Organization Affairs, the people in Washington who will transmit instructions to Young and his colleagues in New York.

Young himself has selected William Maynes, a staff member of the Carnegie Endowment for International Peace who knows the United Nations intimately and served on the Carter transition team, to be assistant secretary state for international organization affairs, according to sources at the United Nations. He will replace Sam Lewis, a career diplomat.

Leonard, who sources here and in Washington say will be Young's chief deputy, is a former State Department disarmament expert who is now president of the U.N. Association of the United States. Leonard will replace career diplomat W. Tapley Bennett Jr.

While it is unusual for a new ambassador to make a clean sweep of all high-level posts in the U.S. mission, an ambassador who comes in when the administration changes hands usually changes the holders of most posts.

Another newcomer will be Don McHenry, a member of the Carter foreign policy transition team who once worked on U.N. political affairs at the State Department and is also on the Carnegie Endowment staff, U.N. sources say.

McHenry, like Young, is black.His appointment would fulfil Young's public pledge to appoint another black on the ambassadorial level at the United Nations.

The reaction at the United Nations to rumors of the purge has been mixed. Some diplomats are pleased by the symbolism of the clean sweep and by the foreign affairs experience the incoming crew already has. They are willing to await the new ideas and approaches of the new groups of American diplomats.

Others, including some Americans, fear that the absence of a holdover diplomat with a knowledge of the technical details of past U.N. meetings and negotiations will prove to be a practical disadvantage for Young.

"You need someone around to show Andy the ropes, where the bodies are buried and how to handle the people he'll have to be with," said one old U.N. hand.

The other career diplomats leaving U.N. jons will be Albert W. Sherer Jr., Jacob M. Myerson, who deals with economic affairs, and Nancy V. Rawls, who stepped into her job only two weeks ago. Rawls and Bennett are both Georgians like Young.

The leading contenders for two other top U.N. jobs also combine foreign policy experience with being outside the foreign service.

Wayne Fredricks, a Ford Motor Co. executive who now serves as a liaison to the federal government and who was a deputy assistant secretary of state for African affairs during the Kennedy and Johnson administrations, has been pushing for the post of American ambassador to the U.N. Economic and Social Council, the senior official dealing with all U.N. economic affairs.