The Reagan administration's change of heart regarding the evils of the Soviet Empire and the wisdom of arms control are well advertised and, indeed, will be on public display during the coming summit. Buried in the presummit enthusiasm, however, is an even more remarkable about-face: the Reagan administration's bailout of the United Nations.

The U.N. is facing financial calamity. Two weeks ago, Secretary General Javier Perez de Cuellar declared that the U.N. would be unable to meet its payroll unless it received at least $87 million from the United States. The Reagan administration has just promised the money.

The United States had been withholding much of its $212 million U.N. assessment because of dissatisfaction with extraordinary profligacy and waste at the U.N. This was most grotesquely symbolized when, in the midst of the last Ethiopian famine, the General Assembly voted $73.5 million for a conference center in Addis Ababa. Congress responded by passing the Kassebaum Amendment, withholding some of the American contribution to the U.N. until it overhauled its budget process.

Last year a limited and tentative reform was announced by the U.N. The Reagan administration quickly declared victory and began lobbying Congress to restore U.N. funding. Congress remained skeptical and stingy. And for good reason. The U.N. has "resisted even beginning the procedure for reform," says Alan Keyes, the former administration official most responsible for the reform push. Reform is "dead in the water," says Keyes. It is "business as usual."

That the United States should have caved in on the issue of U.N. reform is bad enough. Worse is what the bailout will cost. Because Congress appropriates money in one bill for all international organizations, the $100 million promised the U.N. must come at the expense of such institutions as the World Health Organization, NATO headquarters in Brussels and the International Civil Aviation Organization. They actually do useful things in the world: fight disease, protect freedom and regulate safe international air travel.

What does our U.N. subsidy buy? The very week of the American bailout a demonstration is being offered. The General Assembly is considering a Syrian resolution calling for a U.N. conference to redefine terrorism. Current definitions, explained the Syrian delegate, are unsatisfactory. The indiscriminate use of violence against innocents, if practiced by "national liberation movements" (i.e., people of whom the Syrians and their U.N. friends approve), is not terrorism. Resistance and retaliation by "racist" and "colonial" states -- that and that alone is to be defined by the U.N. as terrorism.

Thus the massacres at the Rome and Vienna airports, the attempt to place a bomb on a civilian airliner at Heathrow airport and the blowing up of a nightclub in Berlin are not terrorism. Terrorism is the American retaliatory (and deterrent) raid on Libya.

The Syrian resolution may seem like an Orwellian inversion to you, but that is how the U.N. conducts business day to day. A year ago this week, for example, the General Assembly voted to condemn the U.S. raid on Libya, branded it a violation of international law, called for the United States to pay "appropriate compensation" and made no mention at all of the provocation -- namely, previous Libyan terrorist attacks. That, after all, is national liberation, not terrorism.

The official American response to the current Syrian proposal was embarrassingly weak. It argued limply that the proposed terrorism conference would be "pursuing unnecessary conceptual problems," would be "highly unlikely to succeed" and "could easily divide us and erode the measure of communality now existing" on the issue of terrorism.

Communality? With Syria, regarding terrorism, the United States has no communality. The U.S. delegation could not bring itself to say that. But then again the U.S. delegation found it indelicate to note that Syria brought up the idea of redefining terrorism shortly after German and British courts found that Syria was behind the attack on the Berlin nightclub and the attempted mid-air bombing of the jumbo jet from Heathrow.

Benjamin Netanyahu, Israel's U.N. ambassador, said regarding this resolution that even at the U.N. "there must be a limit to excess, and no less important, a limit to farce." Doubtful. But for the United States the question is: "Is there a limit to American indulgence?" Congress is reluctant to save the U.N. until it begins to demonstrate real reform. The Reagan administration is going to bail it out regardless.

Call it sitting small. Reagan came in pledging not to let the United States be kicked around anymore, least of all by the U.N. The U.N. continues kicking, and Reagan is now in the process of saving it. At least regarding his revisionism on the Soviets, Reagan can argue that we must deal with them in some way. As for nuclear weapons, he can say that something useful might be done with them, such as reducing their numbers. For his rescue of the United Nations, Reagan has not excuse.