The seven boats have received extensive refits during their second and third maintenance cycles, which have been planned to bring the vessels to “almost the same level of that of the latest model [Souryu-class] submarine” while extending their service lives.


http://www.janes.com/article/77511/japan-outlines-submarine-modernisation-efforts


오야시오급부터는 적용되지 않는 이야기임. 훈련함으로 용도 변경된 2척 빼고 소류급 상당으로 개량되서 수명이 딴 나라 잠수함들처럼 30년으로 연장됨.