President Yoon Suk Yeol on Thursday appointed all three top-level presidential secretaries, promoting his chief policy secretary, Lee Kwan-sup, to chief of staff, appointing an economic expert to succeed Lee and tapping First Vice Foreign Minister Chang Ho-jin as his national security adviser.

All of the ministerial-level presidential secretaries, who report directly to Yoon, will start work on Monday, the first day of 2024. Appointees do not have to go through a confirmation hearing at the National Assembly.

The sweeping change was in sync with the reform drive of the ruling People Power Party, whose interim party chairman, Han Dong-hoon, announced a list of emergency steering committee members Thursday, an official of the presidential office said on condition of anonymity

Lee, who has been Yoon's policy chief of staff since November, was appointed as the chief of staff to replace Kim Dae-ki.

Lee, 62, joined Yoon's office as the senior presidential secretary for state affairs coordination in August 2022. He was later promoted to presidential secretary for policy, a position Yoon initially abolished to reduce the team of presidential advisers upon inauguration in 2022, but then revived. Before joining the presidential office, Lee served as the first vice minister of trade, industry and energy from 2014 to 2016.

The move came as Kim is set to resign from the post effective at the end of 2023. Kim was the first chief presidential secretary of the Yoon administration. Kim's offer to resign was accepted by Yoon on Tuesday, according to Yoon's office.

Sung Tae-yoon, an economics professor at Yonsei University, will succeed Lee. With an academic background in finance, international economics and macroeconomics, the 53-year-old was a policy adviser to the Ministry of Finance and the Financial Services Commission, according to Yoon's office.

Lee told reporters that he will work to improve ordinary people's livelihoods and keep close tabs on macroeconomic risks to navigate the way out of "complex crises." Sung pledged to come up with policies to improve the lives of ordinary people.

Yoon also appointed First Vice Foreign Minister Chang as the new director of the presidential National Security Office.

Chang will become Yoon's third national security director as he succeeds Cho Tae-yong, who was nominated to lead the National Intelligence Service on Dec. 19. Cho's parliamentary confirmation hearing is scheduled for Jan. 11.

Chang, 62, was ambassador to Russia after he returned to the diplomatic scene in August 2022, following a hiatus during former liberal President Moon Jae-in's tenure. Chang was promoted to the first vice minister post in April.

The career diplomat took on roles at the Ministry of Foreign Affairs including deputy head of North Korean nuclear affairs in 2005 and director general for North American affairs in 2007.

He served as the presidential secretary in charge of foreign affairs under former conservative President Lee Myung-bak. He also worked as a foreign affairs aide to Hwang Kyo-ahn, who was prime minister during the conservative Park Geun-hye administration.

Kim Hong-kyun, the 62-year-old South Korean ambassador to Germany, will succeed Chang, effective Monday.

Chang told reporters that he would work to address North Korean nuclear and missile provocations and seek normalized relations between the two Koreas through strengthened defense cooperation with the United States and Japan, as well as through extended deterrence and a "three-axis" defense system comprising preemptive systems to take down North Korea's missiles and plans to strike back at Pyongyang in retaliation.

Chang's nomination comes at a time when the National Security Office is poised for reorganization.

Yoon's office announced earlier in December that the National Security Office will create a new division dedicated to supply chain stabilization and other matters related to "economic security," while the existing two divisions will focus on foreign affairs and national defense, respectively.

Chang formerly worked with Kim Tae-hyo, the incumbent first deputy chief of the National Security Office, at the presidential office during the Lee administration.

Also, Chang's appointment is a follow-up of Yoon's nomination of the key posts in the Cabinet's diplomatic and national security front earlier in December.

In the meantime, the Foreign Ministry is experiencing a sweeping change in the leadership positions. Foreign Minister Park Jin has expressed his bid to vie for a parliamentary seat in the April general election in December. Second Vice Minister of Foreign Affairs Oh Young-ju was nominated to lead the Ministry of SMEs and Startups, but the National Assembly failed to reach agreement on whether to confirm her before the suggested deadline. President Yoon has the power to approve his nomination despite the show of disapproval by the National Assembly.

Cho Tae-yul, former permanent representative of South Korea to the United Nations and nominee to succeed Park, will have his confirmation hearing in the assembly on Jan. 8.