The Soviet Union has for the first time modified some aspects of its position at the Geneva negotiations on reducing long-range nuclear missiles. But administration officials say it is not clear whether the Soviets may also turn flexible on the more crucial issues dividing the superpowers at the talks.

The Soviets have reportedly withdrawn demands that the United States deploy no more than four to six new Trident missile-firing submarines, reduce the missiles on each submarine from 24 to 16 and not deploy the new Trident II missile under development for those vessels.

The Soviets also have stopped demanding that jet-powered cruise missiles launched from bombers be limited to a range of 360 miles.

Administration officials say these shifts reflect at least some movement in the strategic arms reduction talks, or START, which began a year ago, in the sense that the two proposals are now slightly less incompatible. The United States has recently altered its START position, too.

On the other hand, U.S. officials say the demands the Soviets dropped were never realistic.

Administration officials view them as "peripheral," as one put it, to what the White House sees as the main objective of the talks: sharp reductions in the large Soviet force of accurate land-based missiles, the only ones that currently have the theoretical capability to wipe out U.S. land-based missiles in a first strike.

The Soviets have said some U.S. demands are also unrealistic: those that would require them to give up 75 percent of their most prized land-based missile force.

First reports of some of the Soviet shifts appeared in Sunday's Boston Globe.

The Reagan administration wants to build 100 new highly accurate MX land-based missiles to help offset this Soviet threat. But the United States is also planning for at least 14 new Trident submarines. The first two now at sea carry the Trident I missile, which is not as accurate as the MX and cannot be used to attack Soviet missile silos on land.

But the Trident II missile, which has not yet been tested and will not be ready until 1989, is supposed to be accurate enough for that job, and Moscow has been trying to block it at the talks.

The Soviets, however, already have tested and now have a new sea-based missile of their own, the SSN-NX-20, which specialists say is probably already operational aboard the new Typhoon-class submarine. So the Soviets have been seeking at Geneva to bar new submarine missiles beyond those already tested, which would have allowed theirs but not the Trident II.

Why Moscow has backed away from this is not clear, aside from the fact that the administration seemed totally committed to the Trident program. Some officials believe the Soviets decided not to try to cut off the submarine force because this could result in deployment of more MX land-based missiles, which are more threatening since they would theoretically have to be fired quickly in a crisis or run the risk of being lost.

Some officials also speculated that the Soviets, with their own land-based missiles becoming vulnerable, may now be more interested in expanding their sea-based force. But others said that the Soviets, without any warm-water ports, were not likely to shift too dramatically to reliance on submarines.

Officials here say the Soviet shift on cruise missiles is in keeping with the U.S.-Soviet strategic arms limitation treaty (SALT II) agreement in 1979. In that agreement, which never was ratified by the United States, no range restrictions were placed on cruise missiles launched from bombers.

The Soviets, however, have apparently not dropped the 360-mile limitation demand for cruise missiles based on ships or submarines, and this could become an important issue at the talks.

The Soviets were also said to have agreed to establishment of a working group to study various so-called confidence-building measures proposed by this country that would involve such things as prior notice of missile test launching or of large bomber or submarine deployments.

Administration officials also said the United States is likely to submit a new START draft treaty in the next week or so at Geneva, incorporating the changes recently announced by President Reagan.

Aside from dealing with the missiles of each side, the new proposal will formally suggest allowing each side 400 bombers and that these bombers be allowed to carry new cruise missiles to be counted under an agreed-upon formula. The United States had previously indicated its intention to do this.