In what seems to be a major reversal of policy, yet one that has attracted little public attention, the Pentagon apparently will take all 10 of its aging Polaris submarines out of its missile-firing undersea fleet well before anywhere near that number of new Trident submarines enters the fleet as replacements.

The assumption had always been that the Polaris vessels, each of which carries 16 missiles, would be replaced gradually as the more modern and powerful Trident submarines, each carrying 24 missiles, enter service. "Polaris submarines will be withdrawn from service as Trident deploys," Defense Secretary Harold Brown told Congress in his annual report in January 1979.

But in the report sent to Congress in January 1980, Brown dropped that sentence and referred only to the retirement of the 10 Polaris submarines by fiscal 1981. The first Trident submarine was slated for sea trials in July 1980, Brown said at the time.

Since 1967, the United States has had 41 missile-firing submarines, including the 10 oldest Polaris vessels and 31 more modern craft carrying Poseidon missiles. In keeping with terms of the U.S.-Soviet strategic arms limitation treaty, the United States is dismantling two Polaris subs to balance the instruction of the first Trident vessel, even though delays in the Trident project have delayed its entry on patrol until early 1982, according to the Navy.

Yesterday, however, James L. Buckley, the Republican senatorial nominee in Connecticut, charged that all 160 missiles from the 10 Polaris submarines are being removed in what amounts to a 25 percent reduction of the missiles-firing submarines force. Buckley claimed that Carter Administration strategists decided this month to do this. It had been known that the Navy was planning to turn the last eight of the Polaris subs from missile launchers into attack submarines, whose role is to knock out enemy subs.

In response to questions about Buckley's charges, a spokesman said the Navy plans to convert all eight remaining Polaris submarines to their new role between now and early 1982, and that the first of these additional deactivations has started. His means that the Navy's missile-firing fleet will drop from 41 to 31 or 32 vessels by 1982 until the first Trident goes on patrol.

Navy spokesmen said it was unclear whether a second Trident would enter the fleet in 1982, so the reduced number of submarines could continue for a long time.

Administration officials, asked about this yesterday, said it was their understanding that the Navy was not "locked in" to this timetable. They indicated that the decision to reduce the Polaris fleet had been reviewed at the White House some time ago but that it had not been reviewed recently, when the new delays in Trident deployment became known.