MOSCOW — The Communist Party Central Committee strengthened Mikhail S. Gorbachev's leadership of the Soviet Union on Friday by approving changes he sought in the ruling Politburo and by endorsing his plan for radical economic reform and his call for an extraordinary party conference next summer.
The committee promoted four key Gorbachev allies in a reshuffle of the Politburo that dramatically weakened the bureaucratic old guard, which Gorbachev has more than once accused of foot-dragging on his program of change.
Obviously pleased at the outcome of the Central Committee's two-day meeting, Gorbachev said the committee had begun, at last, to translate theoretical plans into practical action.
"Ours is a rapidly changing society now," the news agency Tass quoted him as saying, "a society with new attitudes and new hopes. . . . We are in for new problems, considerable complexities. We are not insured against mistakes. . . . Yet I am confident that the greatest mistake is fearing to err."
The Central Committee promoted a key Gorbachev aide, Alexander N. Yakovlev, to full membership in the ruling Politburo after only six months as a non-voting candidate member.
Top Economic Adviser
It also moved Nikolai N. Slyunkov, a top Kremlin adviser on economics, up to full membership, and Viktor P. Nikonov, an agriculture specialist, was made a full member without having served the usual apprenticeship as candidate member.
Defense Minister Dmitri T. Yazov, named to that post only last month, was made a candidate member of the Politburo in place of Marshal Sergei L. Sokolov, who was forced to retire after a teen-age West German pilot penetrated Soviet airspace and landed his light plane near Moscow's Red Square, next to the Kremlin Wall.
These top-level changes mean that the Politburo, which Gorbachev heads, now has 14 full members and six candidate members. Six of the full members hold the title of secretary of the Communist Party, a rank that a Soviet leader usually must have to be considered for advancement to the post of general secretary of the party, the position now held by Gorbachev.
Yegor K. Ligachev, considered the No. 2 man in the Kremlin hierarchy, conceivably could now have four rivals on the Politburo if the top post should become vacant.
In a related development, former Gorbachev adversary Dinmukhamed A. Kunayev was dropped from the Central Committee. He had been accused of "serious shortcomings" as the party's first secretary in the Soviet republic of Kazakhstan. In February, Kunayev was dropped from the Politburo, on which he had served since 1971 as a protege of the late party chief and Soviet President Leonid I. Brezhnev.
1988 Party Congress
In the first such action in 46 years, the Central Committee approved Gorbachev's call for a party conference in Moscow starting June 28, 1988, to review the progress of his campaign for perestroika --economic rebuilding--and additional democratization of the party.
Clearly responding to Gorbachev's request, the committee directed that delegates to the conference be elected by secret ballot at Central Committee meetings in 11 of the 15 Soviet republics. It said that delegates from the other four--Ukraine, Byelorussia, Uzbekistan and Kazakhstan--are to be chosen at meetings of regional party committees.
There was no mention of secret ballots in these four cases and no immediate explanation why delegates from the four republics should be chosen in a manner different from the rest.
At last January's meeting of the Central Committee, Gorbachev recommended secret ballot election of most senior party officials, but the committee at that time said no.
Western diplomats say that the extraordinary step of calling a party conference is an indication of the high priority Gorbachev has assigned to moving swiftly to restructure the management of the cumbersome Soviet economy.
'A Major Political Event'
"This will be a major political event in the life of the party and the country," Gorbachev said in a speech concluding the Central Committee meeting. "For us, Communists, it will become in effect a political exam in the main subject of our life--the reorganization."
Diplomats said the approach eventually taken by the party conference will serve as a guideline for lower-level party officials, who might otherwise tend to delay action on Gorbachev's plans.
Gorbachev said that he wants to avoid a purge of party officials, as long as they are competent and support the principle of overhauling the economy.
"The reform," he said, "will proceed with much difficulty if we do not avoid such a shortcoming as personnel reshuffles in the main section of cadres, such as the leaders of enterprises, construction projects, collective and state farms."