The United States, in a change of position, has agreed with the Soviet Union that all details of a strategic arms limitation treaty (SALT) should be concluded before a meeting between President Carter and Soviet President Leonid I. Brezhnev.

Previously, the United States and said that some decisions-particularly a letter on limiting production and basing of the Soviet Backfire bomber-should be thrashed out by the two leaders at a SALT summit.

But following a meeting yesterday between Secretary of State Cyrus R. Vance and Soviet Ambassador Anatoliy F. Dobrynin, a senior U.S. official told reporters that the plan now is for all elements of a SALT II accord, including the letter on the Backfire, to be agreed upon in advance of the summit.

U.S officials said the switch of position was aimed mainly at denying ammunition to domestic opponents of the SALT agreement. There was concern, the officials said, that critics might charge Carter later with negotiating a bad agreement under the pressures of a summit.

There also was speculation that the change was due partly to Brezhnev's reportedly bad health and a desire to spare him from too much bargaining at the summit. But when State Department spokesman Hodding Carter was asked if Brezhnev's health was a factor, he said "no."

The Carter administration has accepted the Soviet contention that the medium-range Backfire is not a strategic weapon and therefore should not be specifically limited by the SALT II pact, which will impose ceilings on U.S. and Soviet strategic nuclear weapons through 1985.

However, in response to U.S. arguments that the Backfire could reach targets in the United States under some conditions, the Soviets reportedly have agreed to limit its production, deployment and refueling capabilites.

Although the Backfire letter will not be part of the treaty text, it is to be submitted to the Senate as part of the treaty package requiring Senate approval. That was why the administration originally wanted the language of the letter to be worked out at the summit level.

On the other side, the Soviets had favored the idea of negotiating all elements of the treaty in advance and leaving the summit as an occasion for ceremony and informal talks between Brezhnev and Carter.

The officials said the change means that Vance and Dobrynin are now in charge of negotiating the remaining issues, with the detail work of "checking commas" being left to the two countries' SALT negotiation teams in Geneva.

The Vance-Dobrynon meeting yesterday was their 22nd at the State Department this year, and they are scheduled to get together again Thursday. However, Hodding Carter said he is unable to predict when they will have the treaty wrapped up or when or where the summit will take place.

Given the logistics involved in setting up a summit, even an early conclusion probably would preclude a meeting before June. Because of Brezhnev's health, the Soviets want to hold the summit in a neutral European capital such as Geneva, Vienna or Helsinki, rather than making a longer journey to the United States.

Meanwhile, top-ranking administration officials, in a series of speeches to drum up public support for the SALT accord in the tough fight for approval that it will encounter in the Senate.

Vance, speaking in Chicago last night, said the day is gone when either Washington or Moscow can gain a clear strategic nuclear superiority because "each side has the financial and technical resources to keep pace with the other."

For that reason, Vance continued, the rough equivalence that SALT II would establish between the nuclear arsenals of the two superpowers represents "the only realistic strategy in today's nuclear world."

In New York, the president's national security affairs adviser, Zbignew Brzezinski, told newspaper editors that while SALT will not end the arms race, it "will provide for greater stability, symmetry and predictability in the complicated U.S.-Soviet strategic relationship."

Speaking here, George M. Seignious, director of the Arms Control and Disarmament Agency, rejected as "wrong" the charge of critics that the Soviets have superior nuclear forces and want the SALT accord as a means of cementing their edge.

Instead, he asserted, SALT serves the interests of the United States and its allies by demonstrating "that rivals can control their competition" and by acting as "the linchpin" of potential other attempts to control the spread of nuclear weapons.trol the spread of nuclear weapons.