Those who wonder what April in Paris is like will be able to ask Treasury Secretary James A. Baker III when he returns from the annual meeting of the Organization for Economic Cooperation and Development later this month.

The April 11-12 meeting will mark Baker's first foreign trip as treasury secretary. The session will include representatives of 24 industrialized nations and is expected to be a preview of the seven-nation economic summit scheduled for Bonn in May. Baker is to be accompanied by Deputy Secretary Richard G. Darman; Margaret D. Tutwiler, assistant secretary-designate for public affairs and public liaison; David C. Mulford, assistant secretary for international affairs; and Kim Hoggard, deputy assistant secretary for public affairs. TAX REFORM UPDATE . . .

The department is still preoccupied with tax reform. According to a high-ranking Treasury official, the agency's final report, which the president must approve, will be released in late April or early May. Officials say that despite pressure from business and special-interest groups, most of the tax breaks that former secretary Donald T. Regan's proposal would have eliminated will not be restored in the final version.

"You have to make fundamental changes in order to get your rates down . . . ," the official said. "So you're constrained . . . . There have got to be fundamental reforms."

So far, Baker has met with House Ways and Means Chairman Dan Rostenkowski (D-Ill.), Senate Finance Committee Chairman Bob Packwood (R-Ore.), Russell B. Long (D-La.), ranking minority member of Senate Finance, and John J. Duncan (R-Tenn.), ranking minority member of House Ways and Means. Their committees will take up the tax reform proposal when it reaches Capitol Hill.

Although reporters tend to describe the meetings as negotiations, the members have a different view. "Some of them will say we're not negotiating, we're consulting," the official said. "Fine. Whatever name you want to call it is fine with us. We are staying in very close touch with them and with the congressional authors of other tax reform bills so we at least know where people are going to be when the time comes for the president to embrace a particular package."

Before the proposal is sent to the Hill, Baker and his team are trying to learn which proposals the key players will support and what the problem areas are likely to be, the official said.

At a recent meeting with reporters, President Reagan said the proposal he submits will not be "in the form of legislation, but simply as a plan" that the administration will turn "over to them to put into legislation." NEW FACE OUT FRONT . . .

Tim Hoggard is quickly building a reputation around the department -- not only for her expertise in dealing with reporters but for her freshly painted office. Its bright lemon-yellow walls make it a standout among the green and beige offices that line Treasury's corridors. Hoggard recently arrived from the White House, where she was assistant press secretary in charge of liaison with the television networks.

Other recent White House emigres include Cecile (Cece) Kremer, formerly staff assistant in the presidential advance office, now deputy assistant secretary for public liaison; and John F.W. Rogers, assistant secretary-designate for management, who had served as assistant to the president for management and administration. MORE CHANGES . . . Thomas J. Healey has stepped down as assistant secretary for domestic finance. He will be replaced on an acting basis by John J. Niehenke, who now serves as deputy assistant secretary for federal finance.

Charles Powers, a public affairs official who has specialized in tax policy, also has resigned to become an account superviser in the public relations department of Ogilvy and Mather.

Powers' duties will be taken over by Art Siddon, a senior public affairs officer.