China has replaced hundreds of middle and upper level military officers, including half of its military region commanders, in what appears to be an effort to revitalize an Army led by frail old men.

The shift of six military region commanders, with one exception, does not seem designed to purge discredited leaders nor limit Army power. Instead it appears to be a plan to bring younger or healthier men into command. Both outgoing and incoming officers involved in the shuffle appear committed, as most of the Army has been during China's last two decades of upheaval, to the policy of economic construction and social order supported by Vice Premier and Army Chief of Staff Deng Xiaoping.

The change at the top of the Army "is just the tip of the iceberg," said one analyst here. "Hundreds and hundreds of replacements are being made to get younger men into these jobs. Some old commanders are being moved up, some moved to other parts of the country and some will be retired."

Diplomatic and Chinese sources here say much of the Army shake-up seems to bear the stamp of Xu Shiyou, a Politburo member who recently gave up his own post as Guangzhou region commander apparently to work full-time in Peking. He is reported to be taking up much of the burden of running China's military establishment from 77-year-old Defense Minister Xu Xiangqian (no relation), who is in poor health.

At 74, Xu Shiyou is a gruff man from poor peasant stock who appears to remain physically and mentally active. Among Chinese soldiers he has something of a legendary reputation for fending off politicians he does not like and displaying great stamina. "They tell all kinds of stories of his feats of strength and how he can drink people under the table," said one foreign military attache here.

Defense Minister Xu, on the other hand, has reportedly not lost his keen grasp of Chinese military strategy but has begun to tire easily. He did not receive his U.S. opposite number, Defense Secretary Harold Brown, at the airport here last month and missed most of Brown's long meetings with Chinese military officials.

Xu's Guangzhou command has been taken over by Wu Kehua, one of China's most experienced Army administrators and a veteran of Korean War fighting at Pork Chop Hill. Wu moved from the command of the Urumqi region, which borders on Pakistan, Afghanistan and the Soviet Union.His deputy commander is Shenyang. The vacant Chengdu command has been taken by You Tiazhong from Peking.

Recent official press articles calling on Army commanders to work to train the next generation indicate some expected resistance to retirement, which in China usually means a loss of special privileges like a car and driver.

In the military region shifts, the commander of the Wuhan region, Wang Bicheng, has been replaced by a deputy chief of the general staff, Zhang Zaiqian. Wang looked very frail when he greeted Brown in Wuhan and diplomats speculate he will retire or shift to less demanding duty. In the Kunming region, Yang Dezhi has been replaced by Zhang Zhixiu, his deputy.

The sixth regional shift came in Peking, the only place where the slowly diminishing purges of the post-Mao Tse-tung era seem to have had an effect on this latest shakeup in the Army. Commander Chen Xilian, a Mao supporter blamed for a crack-down on pro-Deng Xiaoping rioters in 1976, was replaced by his political commissar, Qin Jiwet. Chen retains for the time being his place on the Politburo and his job as vice premier. There have been persistent rumors that the Shenyang commander, Li Desheng, would also lose his job for his past support of Mao's more radical policies, but many analysts here now argue that Li has made peace with Deng and Xu Shiyou and will stay in power.

Moving younger officers into positions of influence appears part of a plan to prepare China's nearly 5 million-strong armed forces for modern warfare against its greatest potential adversary, the Soviet Union. Defense Minister Xu said late last year: "If we treat and command a modern war in the way we commanded the war in the 1930s and 1940s, we are bound to meet . . . a serious defeat."

One key new appointment in this scheme to shift training away from Mao's reliance on guerrilla-led "people's war" is the promotion of Liu Huaqing to deputy chief of the general staff. Liu has been deputy chief of staff of the Navy, but is also considered an expert on modern military science and equipment.