The Carter administration's senior Middle Eastern expert said yesterday that the next move in the Arab-Israeli peace maneuvering is up to the Palestine Liberation Organization.

"We have no idea what they [the PLO] are going to do or "when they are going to do it, but the ball is in their court," Assistant Secretary of State Alfred L. Atherton told reporters at a breakfast meeting.

President Carter, Secretary of State Cyrus R. Vance and other U.S. officials reported during Vance's recent Mideast tour, on the basis of word from Arab leaders that the PLO is considering a change in its position on the existence of Israel and U.N. Security Council Resolution 242.

Resolution 242 sets out the basic concept of Israeli withdrawal from territory occupied in 1967 in exchange for peace within a secure and recognized boundaries. In the U.S. view, acceptance of the PLO of Israel's right to live in peace as a permanent state in the Middle East. Atherton indicated yesterday that the PLO would also be asked for explicit recognition of Israel's right to exist as a condition for participation in the peace-seeking process.

"This is central issue being discussed by the PLO and the Arab governments," Atherton said yesterday. State Department sources said a decision may emerge from a PLO Executive Committee meeting scheduled for Aug. 25 in Damascus. A previously reported meeting of the Palestinian Central Council, a somewhat larger group, appears to have been shelved.

Atherton reitereated that the United States opposes "any attempt to amend, modify or supersede" U.N. Resolution 242 to make it more palatable to the PLO. He would not say flatly that the United States would veto such a move within the U.N. Security Council, explaining that, as a matter of policy, veto intentions are not announced in advance, but he strongly suggested that the United States would do so if necessary.

Asked why the United States rejects any change, Atherton said the U.N. resolution approved in 1967 is the basis for Arab-Israeli bargaining and itwould be "a very dangerous course" to permit a U.N. debate that could substitute oratory and emotion for diplomacy.

He also acknowledged that the United States is committed, in a memorandum supplied to Israel by then-Secretary of State Henry A. Kissinger in September, 1975, to oppose any change in Resolution 242 that would be "incompatible" with its original purpose.

Rebutting suggestions from columnists and reporters that Vance's mission ot the Middle East had failed and that the chances for diplomatic progress are dim. Atherton said, "This is a long process and we're still in the early stages of it."

He said there was no sign of compromise in Israel's private position on the Key issues of territorial concessions on the West Bank of the Jordan River or creation of a Palestinian entity. At the same time, Atherton said, "I certainly wouldn't say we are stalled or deadlocked."

He said the U.S. hope is that once Geneva negotiations begin, the positions of all parties will be modified as part of the bargaining process.

"You are looking at what are obviously wide gaps on the issues," Atherton told a reporter who observed that most journalists who accompanied vance are pessimistic. But "we are looking at the dynamics of a process . . . One has to assume that the [negotiating] process will change positions," he said.

The United States is "still aiming for a Geneva conference this year," Atherton said. He would not estimate the chances for convening such a meeting, however.

After a meeting with Vance yesterday, Rep. Lee H. Hamilton (D-Ind.), chairman of the House International Relation's subcommittee on the Middle East, said he believes "meaningful negotiations took place" during Vance's mission and tha more can be expected during Arab and Israeli foreign ministers' meetings with Vance in New York at the United Nationas next month. Hamilton said he believes "the diplomatic: option is viable."