When Treasury Secretary Donald T. Regan and White House chief of staff James A. Baker III announced last week that they would be trading places, virtually everyone at treasury was caught by surprise.

Alfred H. Kingon, assistant secretary of treasury for policy planning and public affairs, reportedly first heard the news when he was told to go to the White House for the announcement. Later, Regan called his staff together and apologized for not telling them in advance. He then instructed all of his assistant secretaries to prepare briefing materials for Baker.

Since that meeting, according to a treasury official, the department heads have been walking to the White House to brief Baker and help prepare him for his confirmation hearing, tentatively scheduled for next Wednesday at the Senate Finance Committee.

Meanwhile, there have been few decisions on who will move to the White House staff with Regan, who will stay at the Treasury Department, and who will go with Baker. However, it has been announced that Deputy Treasury Secretary R.T. McNamar will be replaced by Richard G. Darman, an assistant to the president and Baker's deputy. McNamar is in charge of briefing Baker for his confirmation hearings.

In between those briefings, top aides at treasury are renewing their contacts at the White House and with other Republicans around town to line up jobs, one official said.

Neither Baker nor Regan has made public any other decisions on staff changes; some departmental officials said that neither has had the time to make any.

Some of the major policy-making players at treasury whose jobs may hang in the balance are assistant secretaries Kingon; Manuel H. Johnson, economic affairs; David C. Mulford, international affairs; Thomas C. Dawson, business and consumer affairs; Bruce Thompson, legislative affairs; Thomas Healey, domestic finance, and Ronald Pearlman, acting assistant secretary for tax policy.

One decision that was made before the job swap began was the appointment of Roger Bolton to be deputy assistant secretary for public affairs. Bolton had been a consultant for public affairs until the White House finished reviewing his appointment last month. HOLD ON TAXES, TOO . . .

The Treasury Department's tax revision program also is awaiting Baker's arrival before devising options for some of its proposals. Last month Regan said the department was considering altering some of the proposals it unveiled in November, such as new depreciation schedules that provoked anger among some businesses. This week Baker told the "CBS Morning News" that the treasury proposal would be changed, but he did not say how.