President Reagan is preparing to inform Congress of plans to send 20 to 25 U.S. military physicians to El Salvador, administration officials said last night.

Reagan also has decided to replace the U.S. ambassadors to Guatemala and Costa Rica, who are due for regular rotation from their posts, the officials said.

A White House official, who asked not to be identified, said the military physicians are being sent "to assist the Salvadorans for badly needed medical services." He added that "it has been under discussion for some time and consultations are under way with Congress."

Other administration officials said the new medical unit will treat patients as well as train and consult with Salvadoran military doctors.

The sources said the addition of the new medical team will not break the administration's self-imposed limit of 55 military advisers in El Salvador.

"The individuals will not be included" under the "limit of 55 trainers because they are not trainers. They are medical personnel," the White House official said.

Another official said that the medical teams are being sent to provide "humanitarian" aid and "therefore they would not be trainers" in the same sense as military advisers.

"Like any medical team, they will be providing some training, but their purpose is to engage in treatment," said a State Department official familiar with the administration's plans.

That official said the doctors would work in existing Salvadoran facilities and could return to the United States "within the month." He said there will be 20 people--"give or take a few"--in the unit.

The official said that the idea of sending a medical unit to El Salvador was developed in talks with some "more liberal members of Congress" who share the administration's concern about the deteriorating military situation there.

The White House did not announce that Reagan is considering sending the medical unit to El Salvador. Only when questioned by reporters did officials confirm that consultations with Capitol Hill are under way.

Earlier yesterday, Reagan insisted that no change is planned in administration policy in Central America. "Contrary to some reports you may have seen in recent days, we are not changing the policy I outlined to the Congress. Nor is there change in the urgent need for bipartisan support for that policy," he said.

This was the second time in as many days that Reagan has made this assertion. He was responding to reports that White House officials plan to take a new, tougher approach to Central America in the wake of the removal last week of Thomas O. Enders as assistant secretary of state for inter-American affairs and the anticipated replacement of Deane R. Hinton as ambassador to El Salvador.

Enders is being succeeded by Langhorne A. (Tony) Motley, currently U.S. ambassador to Brazil. The Hinton move has not been announced formally.

A White House official said that it is only "coincidental" that Reagan is replacing the U.S. ambassadors to Guatemala and Costa Rica at the same time that Hinton and Enders are leaving their posts. The official said that the changes "would be coincidental rather than part of a shake-up."

The official said the two ambassadors, Francis J. McNeil in Costa Rica and Frederic L. Chapin in Guatemala, are due for rotation.

Another White House official said it is still possible, however, that the two shifts may be related to a broader effort to install new personnel to carry out U.S. policy in Central America.

Also yesterday, Secretary of State George P. Shultz presided over the swearing-in of former Florida senator Richard J. Stone as special envoy to Central America. Shultz said Stone's job will be "to put forward a strong shield" against "the Soviet, Cuban, Nicaraguan axis" so that warring factions in the area can move toward negotiations, economic progress and the development of democratic institutions.

Shultz, in a ceremony before most of the Latin American diplomatic corps and State Department officialdom, praised Stone as "a person of great skill, energy and capacity, humor and drive," and indicated he will need those qualities to deal with "one of the most important and difficult problems" around.

Reagan echoed that theme later, calling for a "new solidarity" in the hemisphere based on economic growth. The president, quoting Shultz, told an audience of business executives at the White House, "We are not against anyone in Central America. We are for the people there."

The president praised corporate members of the Council of the Americas group for providing the "engine of development" in Latin America through their investments, which council Chairman David Rockefeller said comprise 80 percent of U.S. enterprise in the region.

Reagan said Central America is threatened by "global recession" and by "the communist threat to the freedom and independence" of the area.

"More and more people are realizing that Marxist socialism can provide rhetoric, but it doesn't put food on the table," he said. The president then quoted an unidentified head of state at last weekend's Williamsburg summit as saying of communists in his own country who have prosperous life styles, "They talk left but live right."

Stone, 54, plans to leave today on a 12-day, 10-nation "listening tour" of Central America. The trip will begin with El Salvador, an aide said. Stone also plans to visit Costa Rica, Panama, Colombia, Venezuela, Honduras, Nicaragua, Guatemala, Belize and Mexico.

Comparing the difficulty of his task with that of negotiating the Camp David accords on the Middle East, Stone said, "The object is not to score points," but to advance the interests of "those who are not carrying guns: the people."