The Wall Street Journal, forced to turn away advertisers' dollars and its reporters' stories for lack of space, is planning its most dramatic format change in at least 25 years.

Some time next year, the enormously profitable Journal will add a second section. If that doesn't sound like news, consider that the caution with which the paper has approached change of any kind during its 90 years has no equal in the American newspaper industry.

"If we do our job right, nobody will notice," executive editor Frederick Taylor has said of the coming changes.

Journal traditionalists worry that readers won't welcome a second section likely to fall on their feet as they lift it from their doorsteps, and then prove cumbersome for bus and subway browsing.

"Living in New York," Taylor said, "a lot of us are commuters, and we're particularly sensitive to this."

The traditionalists have lost the battle, it appears. No starting date for the two-section Journal has been fixed, but "we're operating under the assumption we will do it," Taylor said, adding a two-section Journal would still be small potatoes in the falling-on-your-feet department. The Washington Post, Los Angeles Times and other major U.S. dailies have been publishing multi-sectional papers for years.

Behind the pending change more than anything else is a predicament that would keep most publishers awake at night: For some time, the Journal has had to turn away revenue.

Advertisers have been coming to the paper, only to be told that they might have to wait several weeks to get their copy and display in the paper.

"There's general agreement that it's not healthy to tell advertisers to wait three weeks or so," Taylor said. In an internal memo to the staff, Warren H. Phillips chairman of the paper's parent Dow Jones & Co., said: "Without additional advertising revenue in the future to cover rising postal costs and other expenses, we would risk weakening the Journal economically over the long run."

Phillips added that the paper would become an uncompetitive advertising medium if it tried to raise these additional revenues solely by increasing its advertising rates.

In 1978, the Journal sold about $115 million in advertising, a 17.3 percent gain in revenue over 1977.

Fred Zimmerman, a former White House correspondent and Atlanta bureau chief, has been transferred to New York to edit the front page of the second section under Taylor.

Though planning for the page is not completed, Taylor said it will concentrate on business and economic subjects, and include columns on topics not now regularly covered by the Journal.

The Journal has been expanding its editorial staff, particularly in its overseas bureaus, Taylor said, noting that as matters stand now, "if we run a foreign story we know it knocks out a domestic story or vice versa." The two-section Journal will give editors at least six more columns to accommodate staff-written reports. The additional columns will boost news apce by roughly 7 percent, Taylor said.

At present, the Journal limits itself to a maximum of 48 pages in its Eastern edition and 40 pages in editions sold elsewhere in the country. In the new format, each of these maximums will be increased by eight pages.

Fall advertising sales look "very good," Taylor said. Although the Journal, like other papers, keeps writing about a recession, no recession is apparent in its ad sales tables.

Circulation is also climbing. Now just 1.6 million daily, it has drawn even with, or perhaps passed, the New York Daily News, for years, the nation's largest-selling newspaper.

The Journal's readers are an elite. According to the company's latest annual report, a survey showed that nearly half of the paper's readers were in top management positions; almost 19 percent were chairmen or presidents of their companies. The household income of the average Journal reader exceeds $52,000.

The risks involved in changing a winning formula are not lost on Journal executives. "We know we have a lot of readers who like the paper the way it is," Taylor said. "One of the reasons we are working so hard on this is because we want readers to say we're giving them something more."