A NEW United Nations secretary-general has been chosen. On Jan. 1, 1992, Egypt's Boutros Boutros Ghali will step into a position that is rapidly becoming one of the most challenging in the world. Remarkable changes are taking place at UN headquarters beside New York's East River.Speaking to the General Assembly, Italy's UN ambassador, Gianni de Michelis, said recently: "As we have seen in the aftermath of the Gulf war, the right to intervene for humanitarian ends and the protection of human rights is gaining ground.... Intervention that is primarily aimed at securing protection of human rights and respect for the basic principles of peaceful coexistence is a prerogative of the international community, which must have the power to suspend sovereignty whenever it is exercised in a criminal fashion." Suspend sovereignty? Historically, the UN has done everything it could to avoid becoming involved in what its charter calls "matters which are essentially within the domestic jurisdiction of any state." Just as remarkable as the Italian ambassador's statement has been the positive reaction to it. Australia's ambassador, Peter Wilenski, put it this way: "We are entering a period where traditional notions of sovereignty are in flux. We're redefining the right of countries to take a role in what was once regarded as another country's internal affairs." Sen. Daniel Patrick Moynihan (D) of New York, who served at the UN from 1975 to 1976, says: "The next stage in international law is to require that the internal conduct of governments meet some international standard." And John Scali, US ambassador from 1973 to 1975, flatly predicts: "Not only will the United Nations have the authority in the future to suspend sovereignty, but to disregard an international border if it believes it is necessary to promote peace." This is pretty heady stuff. Just a few years ago another UN alumnus, George Bush, dismissed its work with this comment: "As 'the last best hope for peace,' the UN was another light that failed." The light has been relit. In this year alone, the world body has launched six peacekeeping missions - the most in its 46-year existence. Over 10,000 blue-helmeted soldiers are currently deployed around the world. In Iraq, UN forces are protecting the Kurds, and UN inspectors are seeking out and destroying Saddam Hussein's weapons of mass destruction. UN-sponsored agreements have been signed to help bring to an end two of the world's longest and bloodiest conflicts - those in El Salvador and Cambodia. The first detachment of an es timated peacekeeping force of between 5,000 and 10,000 recently arrived in Phnom Penh. That operation will be the largest in UN history. A peacekeeping mission is now being considered for Yugoslavia. The current secretary-general, Javier Perez de Cuellar, is receiving well-deserved praise for his work in obtaining the release of Western hostages in Lebanon. Increasingly the UN is playing the role for which it was created in 1945 - that of promoting collective security and settling regional disputes. But for the UN to reach its full potential, many observers believe several fundamental changes are needed. THE Security Council, with its five permanent and 10 rotating members, should be enlarged to include other nations whose weight in the international community warrants representation. Andrew Young, US ambassador from 1977 to 1979, proposes that the country with the largest gross national product in a region be added - for example, Japan from Asia, Germany from Europe, Nigeria or South Africa from Africa, Brazil or Mexico from Latin America. Most observers believe the Council could accommodate around 22 m embers without becoming too unwieldy. Many also argue that the permanent members' veto should be maintained or it would be difficult to get any real business done. A second change relates to what is seen as the UN's bloated, patronage-filled bureaucracy. Wholesale reform is needed to make the UN's agencies and its Secretariat more responsive to the problems - not only peacekeeping but social, economic, and environmental - before them. Ambassador Wilenski suggests, "What we have to do is to remove the interference of member states in appointments within the Secretariat." Finally, the UN should develop a more effective system of conflict prevention. According to Yemen's Ambassador, Abdalla al-Ashtal, "We need what is called preventive diplomacy." That would include, he says, intervening in situations directly "even if it takes troops to stop a war before it starts." The UN Association has recently identified two options for providing the kind of military support the UN will need if it is to respond to emerging crises effectively. In one, member states would agree to provide certain units of their armed forces "on call" to the UN for actions authorized by the Security Council. In the other option, Security Council members would authorize the secretary-general to dispatch UN military observers to any state requesting a UN presence because it felt threatened by invasio n. This expanded peacekeeping role would be expensive. The already-approved operation for Cambodia, for example, will cost more than $1 billion. Still, it's been calculated that the one-day cost of Operation Desert Storm would pay for all UN peacekeeping operations for an entire year. Peacekeeping is a bargain compared to war-fighting. As President Bush said in his September address to the General Assembly Now, for the first time, we have a real chance to fulfill the UN Charter's ambition of working 'to save succeeding generations from the scourge of war. The UN is on a roll - keep it going.