WITH THE REPLACEMENT of Konstantin Chernenko by Mikhail Gorbachev, generational change has finally come to the Kremlin. Among those who look on, the tendency has been to confer a greater openness to reform and accommodation on the New Guard, which presumably, unlike the Old Guard, has not been touched by the dark inheritance of Stalinism. But the ostensibly greater energy, education and ambition of the younger generation, plus its lack of firsthand adult exposure to the rigors of the Soviet past, may yet make its members more formidable competitors, more careless and more prone to risk.

It pays to recall that Konstantin Chernenko himself confounded some of the stereotypes. He was 72 and ill when he took over barely a year ago, known as the man who'd carried Leonid Brezhnev's briecase but had been passed over for the top spot when he died. When Yuri Andropov died, he made it. He then proceeded to deliver more change, in the crucial arena, than anyone had anticipated. Mr. Andropov had pronounced Ronald Reagan anathema and stopped dealing with him. The renewal of Soviet-American talks in Geneva today shows how Mr. Chernenko changed the line.

Who is Mikhail Gorbachev, besides being 54? We know he's a good organizer: the Central Committee promoted him "unanimously" only four hours after Mr. Chernenko died. He has the technical education common among Communist Party bureaucrats. He survived the no-win agriculture portfolio and got good press clippings on undemanding missions abroad. As a secretary of the party machinery and a member of the Politburo, he has lived in the Kremlin's fastest lane. Young-and modern-looking, the Gorbachevs surprise a world accustomed to stout Kremlin seniors. Mr. Gorbachev's public statements reflect themes -- an interest in d,etente Soviet-style,an emphasis on domestic priorities -- associated with the Brezhnev rather than the Androprov line. But his true views and his capacity to operate in the thick-as- thieves Soviet political environment are unknown.

It will be important to learn whether the New Guard, in the person of Mr. Gorbachev, has any intention or strategy to tend to the economic lag, the social rot and the political debility bequeathed by the Old Guard, whose only real area of achievement has been the accumulation of raw power. But it will be well for Westerners not to expect the new man in the Kremlin to deliver them from old East- West cares. This is a major moment for the Soviet Union and therefore, unavoidably, for the United States. It is one the United States can best influence by ensuring that its own policy is fair and firm.