All retired personnel when recalled to active duty shall serve in the grade or rate in which they were serving at the time of retirement.
14 U.S. Code § 2503 - Status of recalled personnel
This provision is desirable because many enlisted men and low-ranking officers may now retire with higher grade which they previously held on a temporary basis. If recalled in the higher grades, they might not be capable of holding same at the time of recall. 81st Congress, House Report No. 557.