WHEN President Bush travels to Japan as part of a Pacific trip next month, he will meet with a new head of government.In parliamentary democracies leadership changes can occur with startling abruptness. Margaret Thatcher's replacement by John Major in Britain last year occurred within a matter of days. Now Japan's Prime Minister Toshiki Kaifu has learned, less than a month before the end of his first term as leader of the dominant Labor Democratic Party, that the party barons want a new man at the helm. Mr. Kaifu's forced withdrawal from office has set off a scramble within the LDP. The most likely successors, former finance ministers Kiichi Miyazawa and Michio Watanebe, lead two of the LDP's major factions. The victor, though, will have to gain the backing of former prime minister Noboru Takeshita, who controls the largest faction. It was Mr. Takeshita's abandonment of Kaifu last week that ended the latter's prospects for reelection in a party conference Oct. 27. Kaifu, who has little political base of his own, was installed by LDP kingpins in 1989 as a Mr. Clean who could polish the image of a party wracked by financial scandal. Fittingly, he made political reform his foremost legislative goal. Kaifu has been regarded as indecisive and bumbling in some areas, such as his handling of Japan's involvement in the war against Iraq; but he has been popular with the people because of his commitment to reform. Either of the two leading contenders is likely to be more forceful and effective than Kaifu, and in some respects this will make for better government. But the LDP shouldn't assume that, after Kaifu's sincere yet thus far unrealized efforts, political and financial reform can be put on a shelf. Many Japanese voters seem hungry for greater openness and accountability in politics. The new prime minister is expected to pursue a consistent foreign policy. Like Kaifu, he will favor strong ties with the United States. And he probably will continue to nudge Japan toward greater involvement in world affairs, such as Japanese participation in United Nations peacekeeping missions. Though with a new skipper, Japan is likely to stay on an even keel. That is reassuring for the world community.