in the early stages of the Eastern front, the Soviet generals were of poor quality as Stalin's purges had resulted in the removal of the Red Army's better generals.

When interpreting the reasons for the early incompetence, owing it up entirely to the purge of high command is a mistake. In fact, there are a multitude of factors that contributed to the initial defense being incompetent. Before I talk about the high command, I'll talk about the other affecting factors.

First, the Molotov-Ribbentrop Pact had placed the soviet border 500 kilometers west of the borders it had held since 1921. Therefore, the fortifications that had been put up to defend the soviet union from invasion were manned to an inadequate degree, and although fortifications had begun to be built on the new border, they were not nearly finished by the time the German invasion began. It was also asserted, primarily by Nikita Khrushchev in his memoirs, that in line with prevalent propaganda of the time, which had stated that not a single piece of land would fall to the enemy, Stalin and Voroshilov (who I'll talk about later) had a massive, massive majority of the active red army stationed on the western border (although, as politically charged memoirs, this source isn't overly reliable).

Now, the importance of the purge is more multifaceted than just removing the generals. Possibly more impactful was the purge of petty officers who commanded individual units, who were remove by the ten-thousands and replaced from a pool of not-necessarily talented, often completely untrained (to be in command, that is) and inexperienced soldiers. The purge had also instilled a deep terror in the military, and ever since Tukhachevsky had been persecuted, the most advanced doctrine became taboo. Any deviation from the orthodox but outdated theories that had replaced the Tukhachevsky theories drew commanders into suspicion and as such was avoided. Modern troops, mainly tank crews, were also very poorly integrated and lacked the ability to coordinate with other units, furthering overall soviet incompetence.

Now, to actually answer your question about how the high command was exchanged. It is not entirely true that this was progressive in the timeframe of the war turning around. In fact, the beginning of this exchange happened during the winter was and had been to some degree in progress since mid-1939, embodied primarily in Zhukov's first major successes, and was more or less over by mid-1942

The first stepping stone toward the new generation of soviet commanders came in the form of one of the border disputes that the soviets had with the Japanese. In the summer of 1939, a village under the administration of the soviet aligned Mongolian People's Republic was seized by the Kwantung army, as it lay beyond the Khalkhin-Gol, a river that the Japanese saw as the eastern border of the MPR. The soviets responded to this border violation by sending a large detachment, led by Zhukov, then a division commander. He almost immediately showed massive competence and encircled a number of Kwantung divisions, leading to ~60,000 captured and killed on the Japanese side, almost triple the soviet killed and wounded. In Zhukov's case, this put him in a favorable position, which we will come back to when talking about the first weeks of Barbarossa.

More important in inducing the change was the winter war. To understand the state of soviet high command at this time, let's jump back to the direct aftermath of the military purge. Of five first marshals of the Soviet Union, 2 had stayed alive. These were Kliment Voroshilov and Semyon Budyonny, who will both be influential, both in positive and negative ways. Firstly, the major issue: both Voroshilov and Budyonny were civil war heroes, who were stuck in the tactics of the Civil War, making them slow to embrace technological advancement. Alongside this, although personally very close to Stalin, Voroshilov was an absolutely awful large scale commander, while Budyonny was somewhat competent but had an unreasonable affinity for cavalry and favored its use to the detriment of the modernization of the red army. Now then. Voroshilov was defense commissar until 1940, so the winter war at the end of '39 was effectively under his command. Due to bad coordination of large scale offensives, incompetent field officers, and badly integrated units with poor communications, and a lack of general military cohesion, the winter war was, in its early stages, a complete failure and a display of the inefficiency of the red army. This changed when Semyon Timoshenko replaced Voroshilov as the commanding officer of the Finnish assault. Soon, the soviets broke through, and Finland surrendered to soviet demands. This was followed up by a relatively honest assessment of the war failures by Voroshilov, who then praised Stalin's genius as having turned the tide of war. He was soon replaced as defense commissar by Timoshenko, and a reorganization drive began, the point of which was abandoning the cult of the Civil War and appointing commanders that had an understanding of modern warfare.

For some prominent commanders, like Zhukov and Konev, this was the point at which they were given major postings. A side note here is that both had avoided persecution during the military purge because they were favored: Konev served under Voroshilov, Zhukov served under Budyonny. Others, like Rokossovsky, weren't so lucky.

It is important to note here that the early struggle was not due to incompetent high command, in fact the most prominent officers, like Zhukov, Konev, and Rokossovsky were in command immediately after Operation Barbarossa began. It was much rather an issue of the first 3 factors I described: poor planning, poor officers, and poor unit integration.

On the topic of Rokossovsky. He was the key exemplifier of the other major factor in the officer exchange: the panic in the first months of operation Barbarossa. Rokossovsky had been arrested under charges of being part of the Tukhachevsky conspiracy, alongside a massive number of officers. When the Germans invaded, it became clear that competent leadership was extremely lacking in the red army. Thankfully, many of the officers that were charged had not been killed, and as such, they were freed and immediately put into positions of leadership. This was Rokossovsky's fate as well. The initial panic also had one more contribution to the exchange. At first, Voroshilov, not completely fallen from power, was put in command of the Leningrad front. He then proceeded to personally lead charges and "spent too much time in the trenches", and so he was recalled and replaced with Zhukov. His removal from power was key to finalizing the transition to a more modern-minded red army leadership.