Looking at this you can see why it might make sense
https://www.defensenews.com/global/a...ing-new-helos/
The lawmaker, who sits on the National Assembly’s Defense Committee, said the research concluded in September that the cost of upgrading 17 of the 43 CH-47D Chinook helicopters would be about 1.35 trillion won ($1.2 billion), which is higher than the estimated cost of 1.22 trillion won for buying new ones.
The upgrade cost is partly driven by the fact that Chinook manufacturer Boeing no longer produces parts for older variants, like those owned by South Korea, so specially ordered parts could prove expensive, Min said, citing the research conducted by the Defense Agency for Technology and Quality, which is affiliated with DAPA.