An article yesterday incorrectly reported that the limit of 5,000 nuclear warheads in President Reagan's strategic arms reduction talks (START) proposal would restrict the U.S. and Soviet Union to about one-third of each superpower's present nuclear arsenal. The proposed 5,000-warhead limit represents a one-third reduction from the about 7,500 nuclear warheads now deployed on ballistic missiles by each side.

The changes announced yesterday by President Reagan in the U.S. position at the strategic arms reduction talks (START) in Geneva with the Soviet Union could be an important step toward eventual agreement by the superpowers on curbing their longest-range nuclear missiles and bombers.

Even this new U.S. position would require vast cuts and shifts in Soviet missile forces, which may be impossible to negotiate. At the same time, it would also apparently allow both sides to keep replacing old weapons.

Yet the president's actions, forced on him in large part by pressure from Congress, suggest a greater willingness to be flexible and take Soviet views into account. They also seem to bespeak new confidence from an administration that sees things going in its favor these days--from bipartisan congressional and allied support on arms to the economy--and, thus, seems more sure of itself on arms-control questions. In the long run, what may be most important about the announcements yesterday is not so much the specific missile numbers, which undoubtedly will change again if real negotiations develop. Rather, it may be in the tone of the new proposals, more positive and practical in addressing what has been a vast gap between the Reagan administration and Soviet Union on arms issues.

A senior White House official who briefed reporters yesterday said that the first criterion in changing the proposal was what made sense militarily, would preserve the U.S. deterrent and discourage the enemy from striking first.

But also taken into account, he said, were "Soviet perceptions of stability, deterrence, their own force structure and modernization programs. In short, we thought let's go into it with something that is realistic, has some prospect of making progress and getting an agreement, not just through our own lenses . . . but through Soviet perceptions as we have learned them."

The president's emphasis, he said, was "not to take a position that was so inflexible that the Russians reject it out of hand."

The president altered in two important ways the position he laid out a year ago when START began.

Reagan originally called for both sides to reduce the roughly 7,500 warheads they have by about one-third to no more than 5,000 and to go down as well to no more than 850 land- and submarine-based missiles. The Soviets now have 2,350 of these and the United States 1,600.

The Soviets countered by proposing a reduction to 1,800 missiles and long-range bombers combined. If the bombers are removed, this leaves about 1,450 missiles.

Reagan now advocates raising the 850 number to some level between 850 and the Soviet figure. That moves toward a compromise and, thus, may be attractive to the Soviets, in that it would mean a less drastic reduction in their forces. Reagan moved in this direction only because a bipartisan commission he appointed called for development of a new single-warhead missile that would be less threatening and a less inviting target than current U.S. and Soviet multiple-warhead weapons. The higher number would make room for these additional smaller missiles.

The commission essentially called on Reagan to correct what many critics view as a mistake in his earlier proposal. Allowing each side 5,000 warheads but only 850 missiles encouraged multiple-warhead missiles; it meant each side would end up with five or six warheads in each silo, and five or six to aim at each silo. This, it was thought, would increase both the temptation to strike first and the likelihood that a first strike might succeed.

That is why the commission, backed by many moderates in Congress, called for the small missile and a lower ratio of warheads to missiles.

The other major action by Reagan yesterday amounts to a rejection of demands by hard-liners in the administration, mostly Pentagon civilians, that the Soviets agree to reduce the throw-weight, or lifting power, of their missiles to a specific point equal to ours. Moscow's missiles are much bigger than the U.S. counterparts, and currently have 2 1/2 times the cumulative lifting power, which determines how many warheads and of what size they could hurl.

Getting the Soviets to agree to this was viewed as impossible, requiring even vaster changes in their forces than the general missile reductions. The president decided instead to retain the indirect limits on throw-weight already in the U.S. START proposal and basically tell the Soviets that if they have any better ideas on how to deal with these concerns, or would prefer to deal directly on what is now called the overall "destructive power" of missiles, the U.S. negotiators are anxious to listen.

Thus, on this question the president declined to toughen his stance as advocated by some. But whether this will be viewed as a step forward by Moscow is very doubtful.

The U.S. proposal continues to contain provisions that only half of the 5,000 warheads can be on land-based missiles, the most accurate kind. The Soviets currently have 5,900 of their 7,500 warheads on such missiles, so that would be an enormous cutback.

Also, sub-ceilings that would limit to 210 the number of the biggest Soviet missiles, the SS17s, 18s and 19s, of which Moscow now has some 600, would remain intact. The Soviets reject this as meddling in their decisions on what to deploy.

The problem here is the thorniest of START. While both sides have large forces, the Soviets have invested most heavily in big land-based missiles while the United States has most of its warheads in relatively invulnerable submarines. But the Soviet missiles are the most threatening because they can carry up to 30 warheads apiece, many times more than their U.S. counterparts, and thus must be dealt with in some fashion, U.S. officials say.

The demand for equal throw-weight was originally contained in what was to have been a second phase of the U.S. START proposal. That phase has been dropped, officials said, with the limits on specific missiles and the offer to negotiate other ideas to reach a level of overall equality replacing it.