F YOU'VE READ anything about Mikhail Gorbachev, you've read that he'll be up against "the entrenched Soviet bureaucracy" whenever he tries to change things in the Soviet Union. The Soviet system really is ossified, as Gorbachev himself has acknowledged. "We will have to carry out profound transformations in the economy and in the entire system of social relations," he said last year. The language is Soviet, but the message is clear: we have to shake things up.

Because of ossification, it's easy to predict that the Soviet Union under Gorbachev won't change -- easy, but wrong. There will be change, both in domestic and foreign policies. There will be a dramatic change in the international image of the Soviet Union. We are about to discover that the sleeping Soviet bear of the last half-dozen years has woke up. The changes could be substantial, and they could start to come very quickly.

In one important way the Soviet Union has changed profoundly just in the last week. Last Monday ended a period of nearly eight years in which the Soviet Union was led by infirm old men. (Soviet officials have revealed that Leonid Brezhnev fell seriously ill in 1977, and never fully recovered; he was succeeded in 1982 by the soon-decrepit Yuri Andropov, who in turn was succeeded by the more decrepit Konstantin Chernenko, who died last week.)

Suddenly there's a new general secretary of the Communist Party who is young (54), well-educated, vital, relaxed and by all outward appearances self- confident. This combination of qualities has never before been seen in a Soviet leader. The young Stalin who took power 60 years ago at age 45 lacked the education, and the relaxed quality. Khrushchev, who was 61 when he emerged as leader in his own right, lacked the poise and self-confidence, and the education. Brezhnev lacked all of those qualities. Gorbachev is a new kind of Soviet man.

His elevation will be followed, as summer follows spring, by the great generational change Sovietologists have been anticipating for a decade. As surely as the selections of Andropov in '82 and Chernenko in '84 signified continuity, the selection of Gorbachev in '85 signifies the beginning of a new era in Soviet history.

A new era does not mean a transformed Soviet Union. It will remain a relatively poor, technologically backward, insecure, overarmed, politically ambitious and troublesome world power. Its domestic economy will continue to be inefficient and cumbersome. Its political style will remain autocratic and arbitrary. There may be some loosening of controls, but the Soviet Union will not become a "liberal" society, nor will it learn to respect human rights.

But it will change. Younger people will move into positions of responsibility all over the country, and they will do their jobs differently, because they are products of an entirely different background than the elders they will replace. In many cases these younger men will be more effective administrators.

Most members of the Brezhnev- Andropov-Chernenko generation were woefully ill-informed about the outside world, history, literature, economics -- you name it. There will still be know- nothings in positions of authority in the Soviet Union, but the younger people will represent a qualitative improvement on their predecessors.

More important, perhaps, the new generation (men -- and a few women -- born, roughly, from 1925 to 1935) will not carry the emotional and political baggage of older officials who survived Stalin's terror and World War II. Those who lived through the Stalin era were indelibly stamped by it; they learned fear, caution, hypocrisy and a formalistic style of bureaucratic and political behavior that has now pervaded Soviet life for half a century.

Some of this has seeped through to younger generations, but there is no way to recreate the equivalent of Stalin and the war for those who were too young to experience them as adults. Why does Mikhail Gorbachev look so much more human than any of his predecessors in the Kremlin? Perhaps simply because he was a 23-year- old student at Moscow State University when Joseph Stalin died.

The old men who had to agree to give Gorbachev the top job understood the significance of their decision. They have been evading it for many years, first by refusing to prepare younger men to move up to top positions, then by hanging on to power with their selections of Andropov and Chernenko to fill the last two vacancies. Andropov finally made a move toward younger people, and gave Gorbachev the training and experience he needed to be a serious candidate for the job of general secretary this time.

The old men in the Politburo (half its 10 members are 70 or more, and a sixth is 67) may not relinquish their power immediately, but they must now acknowledge that their day is coming to an end. Gorbachev can be counted on to reinforce that message by adding younger new members to the ruling Politburo.

If, as Seweryn Bialer reports elsewhere in today's Outlook, Foreign Minister Andrei Gromyko was part of an unsuccessful stop-Gorbachev movement, then his days are numbered. In any case his influence will never be what it was before Gorbachev. Now that the Soviet Union has an attractive, energetic new spokesman, the dour old foreign minister will no longer be an indispensable man.

Diplomacy is an area where there could be sweeping changes in the Gorbachev era. Imagine a new Soviet diplomacy in the hands of Gorbachev and, say, Anatoliy Dobrynin, the longtime Soviet ambassador in Washington, as foreign minister. That would represent an extraordinary departure from the last 20 years. In the place of stiff, unimaginative diplomats and leaders, suddenly the Soviets would have engaging, charming younger men who would make good impressions both on the world's political leaders and -- particularly -- on television.

Moreover, such a new team would be able to cast an entirely new Soviet shadow on the world stage. It is difficult to imagine world public opinion holding two such affable fellows responsible for the invasion of Afghanistan or the crushing of Solidarity in Poland.

Gorbachev will enjoy this advantage even without a new foreign minister like Dobrynin. Acting on the same instinct that led Herblock to give Richard Nixon a shave when he became president in 1969, the world is likely to give the new Soviet leader a chance to prove that he is as sensible and genial as he looks. The ultimate results may be no more significant than Herblock's free shave for Nixon, but in the short term, the Soviets could profit enormously from a honeymoon with world public opinion. Our Great Communicator in Washington may finally have a serious rival.

The lesson of Yuri Andropov -- too little appreciated in the West -- tells us something about Gorbachev's prospects as a reformer, if that is what he chooses to be.

When he came to power in November 1982, after 18 years of committee rule presided over by Brezhnev, it was logical to question how much of a mark Andropov would make on his country. He remained healthy for less than a year. But Andropov revealed qualities of leadership not seen in Russia for a generation, qualities that moved the populace and have made him into something of a mythic figure since his death last year. Andropov's brief reign is now seen as a Soviet version of John F. Kennedy's New Frontier.

Andropov's extraordinary reputation was not produced by dramatic departures from conventional policy. He tentatively launched experimental economic reforms, but these did not touch the lives of many ordinary citizens.

What did move the Soviet public was Andropov's personal austerity, his refusal to indulge any sort of cult of his own personality, his shorter, simpler public statements, and his "discipline" campaign against sloth and corruption. He also made available more food in state stores, and allowed the sale of a new, cheaper brand of vodka (quickly dubbed "Andropovka" on the street).

Andropov did all this as an old, worn-out man who looked ill from the day he took office. Gorbachev could follow the model Andropov set, and benefit from his youth and human appeal, too. This sort of approach to leadership would not solve any of the country's fundamental problems, but it could create a new atmosphere in which, at last, it becomes possible to address those problems directly.

Gorbachev will make an enormous contribution if he manages only to reverse the mood of cynicism and indifference that has taken hold of his country. The Soviet Union is in deep trouble, facing an internal crime wave, sharply declining economic performance, a crisis in health care reflected in plunging longevity statistics and the loss of ideological faith.

Small steps can have large consequences in such a bad situation. Agricultural reforms that encouraged more private production could have a dramatic impact on food shortages. Rule changes permitting a little private enterprise in the service sector -- restaurants and auto repair, for example -- could transform everyday life, to the great satisfication of the public. Reforms of this kind have been discussed for decades, and could be introduced without major disruption.

Systematic pursuit of corrupt officials would also raise morale. A campaign to improve health care, now in a dreadful state, would be enormously popular. Gorbachev could offer inducements to the country's discouraged intellectuals by allowing publication of a few exciting novels or poems, or permitting the release of movies already made that his predecessors considered too provocative to show to the public.

The Soviet Union has been so badly run for so many years that a modicum of intelligent, practical change -- far short of radical reform -- could transform the public mood, and probably result in considerably improved economic performance. We should be ready to see Gorbachev try just such a course of action.

Of course that entrenched bureaucracy does exist. To make the Soviet Union into a competitive, modern power, Gorbachev would have to confront and conquer the bureaucrats' parochial interests. If he tries this, he could easily fail. There is no serious prospect that the Soviet economy can be remade into a technological powerhouse competitive with Japan or the United States. It is entirely possible that Mikhail Gorbachev has inherited power in the Kremlin at a time when the Soviet Union has begun an inexorable decline.

But the long-range trends of history won't be clearly visible for years. Meanwhile, Gorbachev retains a bristling arsenal and a great-power position; his country will continue to be America's most troublesome rival. He himself faces an intriguing personal and political challenge. After a long intermission, the drama of competitive international politics may soon begin anew.