The Reagan administration, in a change of tactics, yesterday rushed to the defense of the United Nations to try to overturn a Senate vote that would slash the U.S. contribution to the international organization by $480 million over the next four years.

One administration official said that the timing of the vote was "embarrassing" to President Reagan, who will travel to New York on Sunday and meet with U.N. Secretary-General Javier Perez de Cuellar.

Reagan will speak to the U.N. General Assembly on Monday, and a senior official said he will "strongly reaffirm" his support for the organization. However, only two days ago, Reagan said that U.S. Ambassador Charles M. Lichenstein "had the hearty approval of most people in America" when he suggested that the United States would not object if the United Nations decided to move its headquarters out of New York.

And the same senior official who said that Reagan would reaffirm his support for the body also said that the president believes there is a "tendency toward a double standard" among U.N. member nations that are quicker to condemn the United States than the Soviet Union.

The official said this is because some of the nations are "frightened" of the Soviets. He specifically expressed disappointment that several nations abstained from voting on the resolution condemning the Soviets' downing of the South Korean jetliner three weeks ago, causing the deaths of the 269 persons aboard the plane, and over the failure of the United Nations to condemn Libyan aggression in Chad.

Earlier this week, White House officials expressed satisfaction with the criticisms of the United Nations expressed by Reagan and Lichenstein, saying that they enjoyed widespread public support.

But when the Republican-controlled Senate voted 66 to 23 Thursday night to cut U.S. funding for the organization, the administration changed course and tried to enlist the help of GOP congressional leaders to reverse the vote.

One official who asked not to be identified said that U.N. Ambassador Jeane J. Kirkpatrick considered both the Senate vote and Lichenstein's remarks, which she had criticized, to be "counterproductive." The official declined to say whether she extended the same criticism to the president.

Some of Reagan's most conservative supporters have long urged that the United States reduce its contribution, which now amounts to a quarter of the U.N. budget. But senior administration officials said yesterday that Reagan opposes such a move and would make this clear when he meets with U.N. officials.

While administration officials said they could block the Senate cuts in a House-Senate conference on the State Department authorization bill, the U.N. amendment's chief sponsor, Sen. Nancy Landon Kassebaum (R-Kan.), said she would fight to keep the cuts.

A final vote had been expected yesterday on the bill bearing the Kassebaum amendment, but was postponed until next week after Senate Minority Leader Robert C. Byrd (D-W.Va.) attempted to offer an amendment demanding the resignation of Interior Secretary James G. Watt.

When asked about withdrawing her amendment, Kassebaum said, "Why would I back off? I was not doing it to make a point. I was doing it to bring about a change."

In an interview with the Associated Press, Kassebaum said she thought she had widespread approval for her proposal in both chambers.

"The American people at this time, having gone through a recession and with $200 billion in budget deficits, really feel we're not getting our money's worth at the U.N.," she said. "It isn't going to self-destruct the United Nations. And if it does, then it was a pretty weak reed."

The amendment would reduce U.S. contributions by 21 percent in the 1984 fiscal year beginning Oct. 1, then by 10 percent in each of the next three years.

But a State Department official said the reductions would violate "binding legal obligations" under U.N. rules and that the United Nations could refuse to accept any U.S. payment for two years, causing this country to lose its vote.

White House spokesman Larry Speakes, responding to a question about the Kassebaum amendment, said the president wants funding for the U.N. restored to the levels requested by the administration in the State Department authorization bill.

Few in the Senate are having second thoughts, however. Sen. John W. Warner (R-Va.) said that while the vote "undoubtedly reflected a feeling of emotion," he was protesting "spiraling costs" and "excessive" U.N. salaries that far outstrip those of U.S. civil servants.

Sen. Don Nickles (R-Okla.) is garnering support for a resolution that would authorize a study of U.N. voting patterns with an eye toward cutting foreign aid to countries that repeatedly oppose the United States.

Nickles "doesn't think we should be supporting all these Third World nations that take our money and keep kicking us in the teeth," spokesman Paul Lee said.

A Republican Senate official said that the conference committee will approve some reduction because "the size of the vote is practically enough to overturn a veto."

This official said that while "you could get 30 votes tomorrow for an amendment saying 'U.S. out of the U.N.,'" many moderates were reflecting Kassebaum's concern that the U.N. budget "is really out of control."

A Democratic Senate aide, however, called the vote "a symbolic thing" that probably will be reversed when emotions subside.