The U.S. Army soon will have, for the first time, more people in its drilling reserve units than on active duty, according to Pentagon manpower chief Lawrence J. Korb.

The reversal in the active-to-reserve force ratio, scheduled to take place in fiscal 1988, has far-reaching policy implications.

The change stems in large part from the Army's decision to freeze its active-duty strength at 781,000 for the rest of this decade to concentrate its resources on the weapons it has ordered under President Reagan's rearmament program.

"This increases your risk but it is cheaper to do," Korb said in endorsing what he called the "historic change" in the Army's force structure. At the same time, he rang some warning bells.

"We cannot go much farther without changing our military strategy," the assistant defense secretary for manpower, installations and logistics said. He cited as an example today's forward-deployment strategy, which calls for keeping troops on front lines in places as far apart as West Germany and Korea.

Korb said that if the need for troops overseas increases while active-duty strength remains frozen, the United States would be faced with several difficult choices.

One would be to station most of the active Army abroad and depend on reserves to fill home-front needs, such as responding to unforeseen emergencies. One problem with this option, he said, would be that soldiers would have no ready-made places in the United States to go after serving their tours because of the shrunken "rotational base."

A second choice would be to retrench and stop deploying so many troops in forward areas of the world, a decision that almost certainly would distress several of the allied governments in the North.

A third option, Korb continued, would be to hire more civilians, both Americans and host-country citizens, to take over more jobs of the active-duty Army overseas. But the manpower chief said there is a limit to how far the United States can go in this direction.

Gen. Bernard W. Rogers, NATO commander, already has called for sending more Army and Air Force troops to Europe to help man the new weaponry being delivered there, including Air Force cruise missiles. Rogers has asked Congress to lift its ceiling of 325,600 U.S. troops in Europe.

By the end of fiscal 1986, the Pentagon has projected 781,000 troops on active duty and 751,000 in the reserves, who drill one weekend a month and two weeks every summer. Of the 751,000, the Army National Guard accounts for 450,000 people and Army reserve units, 301,000.

The crossover is scheduled to occur in fiscal 1988, when the Pentagon expects to have 805,800 people in the reserves and 781,000 in the active duty force. Of the 805,800, the Guard's slice is projected at 477,600 and the Army reserve units, 328,200.

"I have two concerns," Korb said. "One is if the reserve and Guard units can meet their recruiting quotas and the second is the difficulties that may come in trying to deploy the Army's two new light divisions."

The light divisions are comprised of three brigades, one of which is made up of reservists. Korb said the light divisions are designed to be fast-response outfits. However, assembling the reserve brigade's members might slow its deployment or compel the light division to rush to the trouble spot with only two of its three brigades.

When Defense Secretary Caspar W. Weinberger took office in 1981, his blueprint called for expanding the Army by 200,000. Instead, the savings to be gained by freezing the size of the Army and letting the reserves grow proved irresistible to both military and congressional leaders. Korb said it costs about $6,000 a year to support a reservist compared to about $30,000 for an active-duty soldier.

"We shouldn't kid ourselves that for every mission the reservists can do as well as active duty forces," Korb said. It will take reserve units longer to get ready for deployment, the manpower chief warned.

"Will we have given them enough training and can we mobilize them fast enough?" Korb wondered.