President Reagan, in a shift of administration strategy, will express support today in a speech to the Organization of American States for a Central American peace plan designed to end armed conflict in Nicaragua and other nations of the region, White House officials said yesterday.

White House spokesman Marlin Fitzwater said Reagan's speech will "emphasize his support for the Guatemalan Plan," which he has previously described as "fatally flawed." The proposal was initiated by Costa Rican President Oscar Arias.

The plan was adopted by presidents of the five Central American nations, including Nicaragua, at an August meeting in Guatemala City after the leaders rejected a rival proposal submitted by Reagan and House Speaker Jim Wright (D-Tex.).

Administration officials said that Reagan remains suspicious of the leftist Sandinista rulers of Nicaragua but recognizes that he must take a more conciliatory approach if he hopes to obtain even humanitarian aid for the contras opposing the government. Secretary of State George P. Shultz has said the administration will seek an additional $270 million in military aid for the contras.

Officials said that this aid request will be submitted to Congress before the Central American plan's Nov. 7 deadline for a cease-fire, but they said Reagan will also say that the military aid could be converted to humanitarian assistance if the cease-fire is implemented and significant democratization continues in Nicaragua. Ultimately, these funds could be converted into development aid for Nicaragua if the contras are integrated into the nation's political process, these officials said.

Nonetheless, the administration still hopes to keep the contras in place, at least for the next several months, as an "insurance policy" for democratic change in Nicaragua, officials said.

However, Wright told reporters that he can envision no formulation of new military aid for the contras that would pass Congress. Wright added that he "cannot imagine" any new push for contra aid succeeding "unless the leaders of the government in Nicaragua . . . were so foolish as to do something provocative."

The administration's assumption when the peace plan was first adopted was that the Sandinistas would give lip service to democratic reforms and make few actual changes. This view was reflected in Reagan's speech at the United Nations on Sept. 21 when he said, "we will not, and the world community will not, accept phony democratization designed to mask the perpetuation of dictatorship."

Since then, however, the Sandinistas have put political pressure on the Reagan administration by allowing the reopening of the newspaper La Prensa and the Catholic radio station, both of which had been shut down by the government. An administration official said that these actions and Arias' frequently expressed call to "give peace a chance" had forced the change in strategy.

Asked how Reagan could support a Central American peace plan he had described as "fatally flawed," Fitzwater said, "We have said that the Guatemala Plan has flaws in it, but that it is a dynamic plan, and that it can work. It is working in many respects, and we want to go through with it and see what we can do with it."

Wright criticized administration strategy on the basis of a Sunday New York Times story that said Reagan would make new demands on the Sandinistas in his speech today -- including elections before the 1990 date required by the Nicaraguan constitution. Fitzwater said the article caused concern in the White House because it "appeared to set up a straw-man debate that focused on decisions that had not been made, and indeed, will not be made."

White House officials said that Wright was assured by White House chief of staff Howard H. Baker Jr. and national security adviser Frank C. Carlucci that the president did not intend to make these new demands. Fitzwater also took a conciliatory approach toward Wright, saying, "We still feel we are cooperating with the speaker . . . . We still feel like we're on the same course and we want to stay on that course."

Wright also played down confrontation with the White House. Although he accused some in the administration of a policy of "obstructionism" toward the peace process, he appeared to give the president the benefit of the doubt. "I hope {Reagan} is operating in good faith," Wright said.