It is official now that the Presidential office is moving to Yongsan:

President-elect Yoon Suk-yeol on Sunday announced a decision to move the presidential office from Cheong Wa Dae to what is now the defense ministry building several kilometers away, saying he wants his office to move out of a “symbol of imperial” presidency and get closer to the people.
The decision is highly symbolic as Cheong Wa Dae has been used for decades as the presidential office and residence, though its secluded location and the way the president’s work office is located away from those of aides sparked criticism it causes the president to be out of touch with the public.
Yoon said he will move into the defense ministry compound in Seoul’s central district of Yongsan immediately after his inauguration on May 10. On the same day, Cheong Wa Dae will be fully opened to the public, he said.
The defense ministry will move into the Joint Chiefs of Staff building in the compound, he said.
Yonhap
There is going to be some offices dislocated from the Defense Ministry that has the Democrat Party lodging complaints about national security:
Yoon admitted that some of the current occupants, including the defense ministry and the Joint Chiefs of Staff, will have to make partial relocations to make room for the presidential office.
He dismissed the notion that moving their offices will create gaps in national defense, a charge that has been leveled by some lawmakers of the ruling Democratic Party. Yoon belongs to the main opposition People Power Party.
I find it interesting now that the Democratic Party is now concerned about national security after years of advocating for appeasement of North Korea.
It also appears the closure of Yongsan Garrison also played into this decision:
Above all, he stressed the advantages of its proximity to a planned national park.
“Starting this year, the (land of the) surrounding U.S. military base is expected to be returned in stages, which will allow us to swiftly construct Yongsan Park, use the defense ministry building as an office and commune and communicate with the people,” Yoon said.
You can read more at the link, but this move does now make it more likely Yongsan will be turned into a park instead of just more apartment towers.