The weekend was spent saying farewell to a son who is leaving for two years of work at a university in China. That kind of occasion inevitably prompts thoughts of a larger and longer-range dimension than fit comfortably in a journalist's brain: thoughts about the changes that may occur between now and his return in mid-1983.

We dwelled, of course, on family matters, from the plans for an addition to a summer cabin to the status of various brothers' jobs and romances. But, when the farewells were finished, it was hard not to wonder how much the country and the world may be altered by the time we sit down again.

Change is everywhere. The vast society in which he will be living and teaching for the next two years just publicly renounced the wisdom of its founder and unseated his designated heir from the chairmanship of the ruling party -- an upheaval so great that superstitious peasants had to be reassured that it did not necessarily signal the advent of earthquakes.

Meantime, the most important country in Eastern Europe, Poland, just conducted the first secret-ballor, competitive election of its leadership in the history of a Communist state. This extraordinary event, occurring almost literally within sight of the Soviet armies, must send a signal of hope to all the other subjected satellites -- and a shiver of fear down the spines of those who rule the Kremlin.

While this was happening, the leaders of the Western industrial democracies were gathering for their annual economic conference. Four of the seven nations were represented by different men than had met just a year before, and two of the three holdovers head parties that are in serious difficulty at home.

It is not possible to draw any simplified chart of the changes reshaping the Western world. France has installed a Socialist government, with Communists in the cabinet for the first time in more than a generation. The United States has installed a Republican administration, some of whose policymakers are closet Libertarians and some of whom are latter-day Puritans.

In Great Britain, which taught most of the rest of the world both the rules of parliamentary democracy and the customs of civility that make it possible for such a system to work, there has been a conspicuous breakdown of civility and social order. Not coincidentally, a brand-new party coalition of the center seems poised to sweep aside both the old parties, the Tories and Labor, just as soon as the voters have a chance to make the change.

The custom is to say that these governmental upheavals represent the political fallout from the mid-1970s' disruption in world energy markets, and the resulting stagflation that besets most of the advanced economies.

That is probably sound analysis. But the suspicion lurks that there is some deeper force at work, requiring massive adjustments in both the communist and the capitalist worlds. The suspicion is that we may be at one of those hinge points in history, when the old order vanishes and a new system brings new leaders to the fore.

The son who is leaving for China was born 14 years after the end of World War II. When he comes back, it will be just over 16 years until the end of the century. The rulers of today -- in both the East and the West -- are people who were shaped by the last great war. Even those newly arrived in power, like Francois Mitterrand and Ronald Reagan, first made their names in the years before and during that epic struggle.

They are not the people who will lead their nations into the new century. What is at test today is whether the ideas and policies -- the very different ideas and policies -- they represent will set the stage for that inevitable transition of leadership.

Reagan and Mitterrand, in their different ways, have demonstrated a remarkable appeal to the young people of their own countries.But it strikes me that the youngest government, the freshest movement, may be that whose appearance is the most miraculous -- that of Poland. It is also the government whose very existence is the most powerful testament to the enduring thirst for freedom, even in the most difficult circumstances.

That is a lesson both father and son will have reason to ponder and perhaps, discreetly, to discuss by mail, in the two years until we sit down again to talk about the many unimagined things that will have occurred by the summer of 1983.