A changing of the guard is coming at the Pentagon, with President Reagan expected soon to name Gen. Larry D. Welch as Air Force chief of staff and either Vice Adm. Frank B. Kelso II or Adm. Carlisle A.H. Trost as chief of naval operations.

Welch is currently commander of the Strategic Air Command, which is in charge of bombers and missiles, but the former fighter pilot is better known in the Air Force for his work in developing advanced fighter planes and tactics. He will be assuming command at a time when the Air Force is developing a fighter for use in the next century.

Welch is described by his peers as extremely bright, but as an introvert compared to the lawmakers he will be dealing with for money in the years ahead, expected to be austere ones for the Defense Department budget. He will succeed another fighter pilot, Gen. Charles A. Gabriel, who has kept a low profile in Washington during his four years as chief of staff.

Gabriel, who leaves office June 27, had been grooming his former deputy, Gen. Jerome O'Malley, for the Air Force's top job. O'Malley and his wife were killed April 20, 1985, when their plane crashed after overshooting the runway at Wilkes-Barre/Scranton International Airport.

"If ever there was a shoo-in for chief of staff," said an Air Force general yesterday, "it was Jerry O'Malley."

After O'Malley's death, Air Force officers were predicting the new chief would be either Gen. Robert D. Russ, commander of the Tactical Air Command, or Welch.

The new uniformed head of the Navy will succeed Adm. James D. Watkins, whose tour as chief of naval operations ends on June 30. Navy Secretary John F. Lehman Jr. is pushing Kelso for the top job, according to Navy insiders, but some Pentagon officials contend the vice admiral is too junior for the job, and they favor Trost. Kelso commands the 6th Fleet in the Mediterranean and ran the battle group that bombed Libya last month. Trost is commander of the Atlantic Fleet, with headquarters in Norfolk.

In another change at the top, Defense Secretary Caspar W. Weinberger has taken on a new military assistant, Vice Adm. Donald S. Jones. Jones succeeds Colin L. Powell, who was recently promoted to the rank of lieutenant general. Powell will command the Army's V Corps in Frankfurt, West Germany.

Lehman recently sent a message to almost every major command in the military twitting Gen. Bernard W. Rogers, the commander of U.S. forces in Europe, for recommending that civilian secretaries like Lehman be abolished and their powers transferred to theater commanders. Lehman in his message said Rogers' recommendation "calls to mind the quote from the recent Gridiron speaker: 'Power corrupts, but absolute power is really neat.' "