A major shake-up at the Justice Department may be resolved as early as today with expected promotions for Assistant Attorney General Stephen S. Trott, chief of the Criminal Division, and other key officials.

Speculation began in earnest yesterday when it was disclosed that Deputy Attorney General D. Lowell Jensen is planning to accept a federal judgeship in California. That would create a vacancy in the No. 2 job.

Department sources are virtually certain that William F. Weld, the U.S. attorney in Boston, has been tapped to take Trott's job as head of the Criminal Division. That would leave Trott in search of a new assignment. But ranking Justice officials have gone out of their way to deny that Trott would ascend to the No. 2 job.

The latest scenario goes like this: Trott may become associate attorney general, the No. 3 job, and the incumbent, former New York lawyer Arnold I. Burns, would move up to be deputy attorney general.

Burns, who built a large Manhattan law practice from scratch and has handled almost every kind of civil case, would be an interesting choice, in part because he has been on the job less than three months. Unlike Jensen, who has worked with Attorney General Edwin Meese III since the 1960s, Burns is not part of the California crowd.

Trott is a respected career prosecutor and former U.S. attorney in Los Angeles who has taken an aggressive approach to espionage and terrorism cases. He has made waves by using subpoenas and forfeiture laws to gather evidence from attorneys who represent drug dealers.

Weld is cut from similar cloth and has made national headlines by prosecuting New England banks for money-laundering violations. He is currently in Washington to coordinate the inquiry into whether an independent counsel should be named to probe allegations that administration officials acted improperly in the 1983 Environmental Protection Agency controversy.

An early rumor had Trott moving across the street to run the Federal Bureau of Investigation. But Director William H. Webster has no plans to step down before his 10-year, nonrenewable term expires in 1988, a spokesman said yesterday.

When important people leave, their aides have to find work. So Jensen's deputy, Jay Stephens, will become deputy White House counsel.

The only other official whose fate is settled is Grover Rees III, Meese's special counsel for judicial selection. After spending a year helping pick conservative nominees for the federal bench, Rees will himself be donning the robes -- as chief justice of the high court of American Samoa. The 34-year-old former Texas law professor, reportedly concerned about charges of favoritism if he sought a domestic judgeship, decided instead to apply for the Pacific post, an Interior Department appointment.(He will be replaced by Civil Rights Division aide Steven Matthews.)

Rees follows the same career path as Kenneth Starr, who advised former attorney general William French Smith on judgeships before moving to the U.S. Court of Appeals here.