The BR change at that time did not result in a decompression of the Air, only a reduction of the top BR from 11.7 BR to 12.0 BR in January or February 2023 which didn’t give benefits for Korean War era jets.
It should not face against least late flareless subsonic jets and should move to at least 10.3BR, since its 2.5km forward is the deathzone for flareless jets.
That is what ended up happening due to the decompression in the ground, and you can see that some 9.7+ vehicles went up 0.3BR.