The United Nations is undergoing a major changing of the guard in its administrative ranks this year, which, combined with a threat to its fiscal stability, could make 1986 a watershed for the organization.

The major changes involve three crucial posts:

*Last week, Brian Urquhart, long viewed as the paradigm of the international civil servant, retired as chief of the United Nations' peacekeeping operations after a career of 40 years, dating back to the founding days of the institution. His successor is Merrack Goulding, a fellow Briton, who is a product of the Foreign Office rather than a U.N. career employe.

*In May or June, Bradford Morse, an American who has run the U.N. Development Program for a decade and also coordinated the African famine relief effort during the past year, will retire.

William H. Draper, III, president of the U.S. Export-Import Bank, has been announced as the American candidate to replace him at the helm of the United Nations' major funding conduit.

*At the end of 1986, the Security Council (and then the General Assembly) must grapple with the question of who will succeed Javier Perez de Cuellar as secretary general when his five-year term expires on Dec. 31. He has made it clear that he will not put his name forward for a second term, but most diplomats believe he can be drafted.

The contract of another American, William Buffum, the undersecretary for general assembly affairs, also runs out in 1986. Buffum's division controls sensitive U.N. operations, such a human rights and Palestinian affairs, but he is likely to stay on until his shepherding of the 1987 U.N. drug conference is complete, then to be replaced by another State Department appointee.

The job held by Urquhart, who has been nominated for the 1986 Nobel Peace Prize for his years of U.N. service, was held by Nobel Laureate Ralph Bunche before him and long has been viewed as the United Nations' political heart.

But the superpowers' current preference for solving problems outside the United Nations (if at all) has cast doubts on the future of U.N. peace-keeping.

Twice in the recent past, in the Sinai in 1973 and in southern Lebanon in 1978, the big powers and the local squabblers reached a point of crisis at which a U.N. force was the only logical solution. Both times the details were tossed into Urquhart's lap, diplomats said, because he was universally trusted to keep confidences, react coolly and quickly to events and operate without political bias.

Some diplomats and U.N officials questioned whether, in the event of a similar crisis, a Syrian or Soviet ambassador could recommend to his capital that control on the ground be given to Urquhart's successor, a product of the British Foreign Office who, as one Arab diplomat pointedly noted, is a graduate of the Shimlan Arabic-language school in Lebanon. That institute, the diplomat said, was perceived in the Arab world as a training ground for British intelligence agents.