MANILA -- A controversial changing of the guard in the Philippine armed forces has elevated a new generation of commanders intent on crushing a long-running Communist insurgency and assuming greater responsibility from U.S. forces for the country's defense.

In one of several recent changes in the military, President Corazon Aquino this month named Maj. Gen. Lisandro Abadia armed forces chief of staff, promoting him over 42 officers with greater seniority. The move prompted a public outburst by one general who had been passed over, and it threatened to split the upper ranks of the 105,000-member armed forces.

Although Abadia, 53, has moved quickly to assert control over the often fractious military, the debate over his appointment has drawn attention to the role of politics in military and police promotions.

Philippine military officials and foreign diplomats generally regard Abadia as a competent army officer and discount the possibility that his promotion could trigger a new coup attempt against Aquino, who has survived seven military revolts in five years. But they express uncertainty because of long-simmering military unrest.

"The bulk of the officer corps is supporting or tacitly accepting Abadia," a diplomat said. "But there are elements that want to exploit this. There are resentments out there."

Since he assumed the top position, picking up a third star, Abadia has vowed to keep the military neutral in national elections scheduled for May 1992 and to achieve a "strategic victory" over the country's 22-year-old Communist insurgency by the time Aquino completes her term. She has pledged not to run for a second term.

Abadia has said such a strategic victory would entail reducing the size and effectiveness of the guerrilla forces, now estimated at 17,000 men, to a level at which their activities could be controlled by police. Some analysts have described Abadia's timetable as optimistic.

Abadia also has promised "internal reforms" in the military, including a drive against corruption. He has proposed to carry out a "modest modernization," with new Italian jet trainers for the air force, U.S. fast patrol boats for the navy and British amphibious armored vehicles for the army.

"As we wind down the insurgency problem, we will have to look beyond our shores to defend the country against aggression," he told reporters.

In replacing retiring Gen. Rodolfo Biazon as chief of staff, Abadia jumped over two top commanders, Maj. Gen. Guillermo Flores and Maj. Gen. Alexander Aguirre. Flores was considered too passive and Aguirre was ruled out in part because he rose through the ranks of the Philippine constabulary rather than the army, government sources said.

Aquino this month also promoted Abadia's older brother, Brig. Gen. Loven C. Abadia, 54, as commander of the Philippine air force. She earlier had named Brig. Gen. Arturo Enrile, 50, to replace Lisandro Abadia head as of the Philippine army.

The changes were seen as an effort to reach out to younger officers and to stem protests by renegade military factions such as the Reform the Armed Forces Movement and the more radical Young Officers Union.

In a joint press conference last week, Abadia and Aguirre indicated they would end recent disputes, and they called for armed forces unity. Aguirre said he would not seek to block Abadia's appointment during confirmation proceedings.

The reconciliation came after the Young Officers Union had declared support for Aguirre. Aguirre said he would continue to push for reforms in the military promotion system, but intended to keep any protests within legal bounds.

On April 8, Aguirre had angrily resigned as armed forces deputy chief of staff to protest Abadia's promotion, accusing Aquino of unfairness and incompetence. In an open letter to Aquino, Aguirre complained he had been "bypassed four times" for promotion under her administration. He said he had been one of her "original" backers when she took over in a "people power" revolution in February 1986 and was the only general wounded in resisting a December 1989 coup attempt.

The day after Aguirre's protest, suspected military rebels bombed an electric power substation and three bank branches, injuring three people.

Among Aguirre's backers has been the Association of Generals and Flag Officers, which represnts more than 400 mostly retired senior military officers. The group expressed sympathy for Aguirre in an open letter and called for reforms in promotions "to insulate the military from partisan political interest."

The issue surfaced last year when Maj. Gen. Cesar Nazareno was chosen over Aguirre to head the 120,000-member Philippine Constabulary-Integrated National Police, the country's largest service branch. Nazareno was regarded as a favorite of Aquino's younger brother, Jose "Peping" Cojuangco.

In a major reorganization, the constabulary and police were separated from the armed forces in January and reconstituted as the Philippine National Police under civilian control. Nazareno remained in charge with the new title of director general.