Sunday's leadership change in Afghanistan and the beginning of a new round of U.N.-sponsored indirect talks on the question of a Soviet troop withdrawal reflect a positive Kremlin assessment of its achievements during the six-year-old war with Moslem guerrillas, according to Soviet foreign policy analysts here.

The replacement of Babrak Karmal with 39-year-old former secret police chief Gen. Najibullah effectively begins a new phase in the Soviet-Afghan relationship, which will be characterized by a gradually diminishing Soviet profile in the region, but not a loosening of Kremlin control over Kabul, the analysts said.

The discussion of a pullout of the estimated 115,000 Soviet troops in Afghanistan indicates that Moscow considers its security interests in the region attainable, despite reports of stagnation in the fighting, poor troop morale and no victory in sight.

Najibullah is expected to continue the military drive against anticommunist insurgents and to pave the way for an era without a heavy Soviet presence, they said.

Najibullah's strengths appear to respond directly to the middle-term political agenda identified in Moscow and Kabul in Babrak's last few months in power, including the need to broaden the governmental representation to include noncommunists, to launch a popular appeal to disaffected Afghans and to carry on with the fighting on the ground.

The Soviet news agency Tass, in announcing Najibullah's selection, said that he was "put in charge of the difficult and decisive sectors -- the party's organizing work and military questions."

As a former chief of security, Najibullah inevitably brings a knowledge of Moslem opposition factions and some experience in isolating them. He also brings experience as a political missionary.

As a relatively young leader, he is deemed more likely by western analysts in Moscow to enjoy popular support than Babrak.

But Najibullah's close associations to the Afghan military, underlined in Tass' brief biography of him, would allow the Kremlin to keep a grip on the fighting with a reduced Soviet troop presence, according to western analysts here.

He made an "immense contribution" to the modernization of the Afghan Army, Tass said.

Unlike Babrak, Najibullah was not closely linked to the 1979 Soviet invasion. And unlike Babrak, he was installed in a bloodless manner, with his predecessor remaining in the government. But he met Soviet officials here during the Communist Party congress in February, and, according to unconfirmed diplomatic reports, returned with Babrak in his visit here last month.

And the official Soviet reports of his election have stressed his allegiances to Moscow. "He is known as an internationalist," Tass said in its brief biography, "a great friend of the Soviet Union."

Still, his selection was designed to appease Mideast and Asian interests, as well as domestic and Soviet requirements, according to western and Asian envoys here.

Babrak spent the month of April in Moscow, but the visit went unreported in the Soviet press.

Four days after his return, he was replaced by Najibullah.

The beginning of the talks on Soviet troop withdrawal corresponds to broader Soviet media coverage of the fighting in Afghanistan, and more frank newspaper probes into war-related issues such as the problems of Afghan veterans. "It is being cast more as a struggle which has dimensions Soviet people can grasp," one western analyst here said.

"It's not a novel," one veteran's mother was quoted as saying in a reference to the war in a May 4 article in the official newspaper Sovietskaya Rossiya.

"Because in these minutes," she added, "you can grasp that perhaps your son will come back from there, and perhaps he won't."