Administration officials and U.S. analysts said yesterday that they foresee little immediate change in the Soviet Union's arms or other policies because of the succession there, but that a younger, more vigorous and probably longer-lasting leader in the Kremlin may eventually bring important shifts.

President Reagan, setting the tone for the cautious U.S. reaction, pledged yesterday to work "with an open mind" with the Kremlin's new leader, Mikhail Gorbachev, in pursuit of arms reductions and improved relations between the two nuclear superpowers.

Reagan, speaking at a White House luncheon for out-of-town journalists shortly after deciding not to make the trip to Moscow for the funeral of Soviet President Konstantin Chernenko, said he would be "more than ready" to meet Gorbachev when he is settled into the job. "We must have an agenda, and not just get acquainted," said Reagan of a possible meeting.

In a condolence message to acting head of state Vasily Kuznetsov, Reagan said, "Although the problems which divide our countries are many and complex, we can and must resolve our differences though dialogue and negotiation." Reagan said the two arms negotiations delegations sitting down today in Geneva "must seize the opportunities for peace." He added, "We must also establish a working relationship that builds greater trust and cooperation between us."

Reagan, in his remarks to the journalists, suggested that he does not expect early change in Soviet policies. "It is collective . . . . The government basically remains the same group of individuals," he said.

Secretary of State George P. Shultz, speaking to the same group of journalists, said the United States will be working to pursue "a continuation of the constructive trend that has been in place now, perhaps hesitatingly . . . symbolized and made concrete by the agreement for the resumption of arms control talks."

Gorbachev, said Shultz, "seems to be a dynamic, strong person." He added, "I do have the feeling . . . that decisions that have been made in recent times have been collective decisions," and "presumably Mr. Gorbachev was part of that process . . . . He is not sick; he is a vigorous young man."

U.S. intelligence reported that Chernenko had little role in policy decisions after his ailments became more serious in January, according to an informed administration official.

Gorbachev, according to a State Department source, has been acting as No. 2 man in the Kremlin since Chernenko assumed power last February, and increasingly has been called upon to take the top role in Chernenko's absence in recent months.

Evidence of this, the official said, was information from Soviet sources that Gorbachev had chaired Politburo meetings and meetings of the Central Committee secretariat when Chernenko was absent.

Another U.S. official said that, for some time, Gorbachev has been "really running everything, and building his base" of authority within the highest circles of the Communist Party.

The new Soviet leader was described by administration sources as "somewhat more willing to accept innovations" than Chernenko had been. On this point, the new leader is described as more likely to resemble Chernenko's predecessor, Yuri Andropov, who sought major changes in personnel and practice in the months before he became ill.

But unlike Andropov, who came to the top job with immense personal authority because of his previous leadership of the Soviet intelligence and secret police organ, the KGB, Gorbachev is believed to have gained the support of others on the Politburo through careful negotiation and a "nonthreatening" demeanor. This suggests that he will have to move cautiously.

U.S. government analysts of Soviet affairs cited Gorbachev's 1983 trip to Canada and his trip to Britain last December as indications of his interest in expanding ties to the West. They cited his close contacts with Hungarian leaders as suggesting an interest in experiments with a mixture of Marxist and market economics, such as have been taking place in Hungary.

The Kremlin's instructions to its nuclear and space arms negotiators are believed to have been essentially set for several months under the dominant guidance of Foreign Minister Andrei A. Gromyko, who has been acting with growing authority in the foreign policy arena.

The Politburo, at its usual weekly meeting last Thursday, approved the details of the instructions to the Geneva negotiations, it was announced at that time in Moscow. But U.S. officials said they assume this was a formality rather than a sign of serious new decision-making.

Because the instructions to the negotiators have been set for some time and because it will take Gorbachev months or even years to cement his authority, officials said, the opening positions to be presented by the Soviet side in Geneva in the next few days are unlikely to be altered in major fashion for a long time.

Secretary of Defense Caspar W. Weinberger, addressing the Veterans of Foreign Wars here, said, "My own personal opinion is that Chernenko's death will not affect the talks very much or the Soviet policy.

"The Soviets are a collective society, a collective government, and I think it is likely that this possibility of his death was taken into account when the various instructions have been given to the Soviet negotiators. I would suspect that they will go directly on with carrying out those instructions, and we will know what those are as the negotiating sessions unfold," said Weinberger.

Senate Majority Leader Robert J. Dole (R-Kan.) and Senate Minority Leader Robert C. Byrd (D-W.Va.), in a statement on behalf of a delegation of senators observing the Geneva talks, said, "We welcome the decision of the Soviet government to start the arms control negotiations on schedule, and we hope that this transition in the Soviet Union will have no substantial impact on these important negotiations as they proceed."

Former secretary of state Henry A. Kissinger, one of dozens of experts interviewed on television, said Gorbachev will need two to four years to consolidate his power. Eventually, Kissinger said, Gorbachev's youth will lead to a realignment of power, but that does not necessarily mean that new policies will be more favorable to the United State.

"Nobody really know what the younger generation thinks in the Soviet Union," Kissinger said. "We don't have any contact with them."

Former secretary of state Alexander M. Haig Jr. said Gorbachev's assumption of power "would probably spell some very important changes in the older hierarchy," including Gromyko and others much older than the new leader.

"Then there will probably be a long period of transition to adjust to the new generation," Haig said.

James R. Schlesinger, a former secretary of defense and CIA director, said that until Gorbachev asserts himself, Gromyko will be "in an even more powerful position."

Former secretary of state Cyrus R. Vance said he looks to a "period of collegial leadership that will last at least a year or so, after which the new leader will begin to assert himself."

On this point, former secretary of defense Harold Brown said, "It will be more like three years or so for the new leader to establish himself in a position of preeminence."

Former White House national security affairs adviser Zbigniew Brzezinski said Gorbachev was groomed through party ranks by Andropov, who headed the security apparatus, and by the late Mikhail Suslov, who had been guardian of party doctrine.

"I look for a more skillful, energetic and, in many respects, a much more dangerous Soviet leader," Brzezinski said.

There was comment within and outside the government about how swiftly the decision on Gorbachev's assumption of power was made and announced.

Several officials made the point that the Soviet leadership has had months to prepare for the decision because of Chernenko's declining health, and that after two previous successions in 28 months the leadership may have become more adept at dealing with the issue.