Newly named Defense Minister Carlos Eugenio Vides Casanova told reporters today as he prepared to take over the most influential post in the government that he will make major changes in the Salvadoran high command to improve its faltering fight against leftist guerrillas.

"We have to see who is doing the job and who is not doing the job and then we have to bring in the appropriate people," said the former commander of the National Guard. But he cautioned that he thinks the war "will go on for a long time."

An end to cronyism and a housecleaning at the top of El Salvador's command structure were the central demands of the officers whose intrigues, mutinies and near-mutinies led to the removal yesterday of long-powerful, highly political Gen. Jose Guillermo Garcia as defense minister.

"We need someone with his eyes looking at nothing but the war, not political things," one senior battlefield commander said last week.

Such officers hope that the new defense minister will promote and employ U.S.-advocated antiguerrilla tactics based on small unit patrols that Garcia appeared reluctant to pursue. But one counterinsurgency expert who has studied the Salvadoran Army closely said this morning, "Any celebration would be a bit premature."

Whatever Vides Casanova's intentions, he is taking over an institution led by independent, often stubborn commanders who have proved remarkably resistant to change in their hierarchical structure. U.S. military advisers often call them "warlords."

There is, moreover, a limited number of men from whom to choose in changing the approach to the war that has seen the government steadily lose ground to the guerrillas during the past several months.

There are more than 400 officers on active duty in the Salvadoran armed forces. Of these, about 60 are currently in positions of significant influence. They are in effect members of an elite club and they are in a position to make or break the war effort, but they often concentrate their attentions instead on making and breaking each other.

In the past six months, and particularly in recent weeks as the maneuvering to oust Garcia intensified, the plots within the armed forces reflected the heavy strains that had been put on the rigid system that has grown up here.

In the past, even in more peaceful times, such discontent often led to the toppling of entire governments. But the officers seeking to remove Garcia were consistently careful to leave the position and at least the nominal authority of civilian interim President Alvaro Magana untouched.

Diplomats here say they believe this is because many officers are aware that essential U.S. aid depends on the visible development of democratic institutions in El Salvador.

Much of the maneuvering in the Army was played openly to the international press and particularly to U.S. opinion, although the American Embassy here repeatedly said it was staying clear of the conflicts.

There were rumblings in the military from the time of a six-day mutiny in Cabanas province in January, but the latest crisis began in earnest last week.

To end the Cabanas mutiny several friends of rebellious Lt. Col. Sigifredo Ochoa, including Air Force chief Col. Juan Rafael Bustillo, had mediated among him, Garcia and Magana.

An agreement was reached, according to Salvadoran military and civilian officials, in which Ochoa would be seen to obey Magana's order reassigning him to Washington on the condition that Garcia step down within three months.

On March 18, as the deadline approached, Garcia submitted his resignation to Magana at a meeting of the country's top commanders. Virtually none of Garcia's fellow officers spoke in his behalf at the time, according to these sources.

The deadline of April 11 passed with Garcia still in place. Two days later Bustillo told two American reporters that if Garcia did not resign by April 15 he would simply stop obeying orders from the high command.

The next day the Air Force chief met with Magana, who showed him the signed acceptance of the resignation and Bustillo left apparently content. But overnight, according to one military source close to Bustillo, he had second thoughts about Magana's sincerity and on Friday he was threatening once again to mutiny.

Yesterday, after a weekend of more maneuvering and negotiations, Garcia did give up his post.