English Translation
He is a soldier turned teacher.
軍隊（ぐんたい）上がり is also frequently used. - I personally prefer 軍隊上がり over 軍人上がり.
上がり in this sentence means “to have finished” or “to have completed” (duties etc.).
Another example with 上がり is 刑務所上がり（けいむしょあがり）, which means “an ex-convict” (those who served prison time). The nuance of 刑務所上がり is “graduated from the detention center”. We don’t usually say 犯罪者上がり（はんざいしゃ）. 上がり is more frequently used for an organizational system or for a systematic rule.