Two weeks before he took the oath of office, President Reagan made Senate Democrats and the people of Japan happy by announcing he wanted former Montana senator Mike Mansfield to stay on as the U.S. ambassador to Japan.

But most of the other political appointees in the nation's ambassadorial ranks apparently will not be so lucky. Some are already on their way home, and the others will be following shortly.

"All the political ambassadors will be replaced," according to a State Department official. "Some are in the process of returning now." Others will wait a bit longer, apparently because of situations in their host nations that dictate the need for a U.S. ambassador in residence.

But about 70 percent of the United States' 120 or so ambassadors to other nations are career diplomats, so the endangered ambassadors number fewer than 40.

Among the soon-to-be-ousted is Sidney Rand, U.S. ambassador to Norway, who told friends back in Minnesota that he had been told to prepare to return to this country within two to three weeks.

It was a short tour of duty for Rand, who was appointed to the post in Oslo in mid-December last year by President Carter on the recommendation of Vice President Mondale. Minnesota Gov. Albert H. Quie and Sen. David Durenberger (R-Minn.) have urged Reagan to reappoint Rand, a former president of St. Olaf College in Northfield, Minn.

But if Reagan vetoes that -- and it appears likely he will -- there are other candidates for the post from Minnesota, with its preponderance of residents of Scandinavian extraction. Sen. Rudy Boschwitz (R-Minn.) has said he will work to have former Concordia College president Joseph L. Knutson appointed to the post, and former Minneapolis mayor Charles Stenvig has expressed interest.

The suite of rooms in the White House annex where Esther Peterson and her consumer office staff worked for the Carter administration was vacant and silent last week. The file drawers have been emptied. The telephones connected directly to the office ring unanswered.

"It's like we don't exist," Peterson, former special assistant to the president for consumer affairs, said from her home.

But Reagan administration officials say the office will be reopened when a replacement for Peterson is found, and Elizabeth Dole, the Reagan aide responsible for the office, said it will be no less important under Reagan than under Carter.

Consumerist Ralph Nader and others have publicly worried that, even if Reagan elected to keep the office, he would name a "do-nothing" director. No way, says Dole, who not only includes the office in her bailiwick but also has some experience in it herself (she was deputy to Virgnina Knauer in the office for a period in the second Nixon administration).

"It will not be downgraded," she said. "The new person will have the same access to the president that Esther Peterson had."