Each service has a Board for Correction of Military (or Naval) Records. These are civilian boards, operating under the authority of the service secretary. They are empowered to correct nearly any error in a current or former service member's record, including being passed over for promotion, appealing and adverse OER/NCOER, appealing a letter of reprimand, Article 15/NJP, etc. The Board for Correction of Military Records can also order a stand-by promotion board for service members denied promotion.
It does not matter whether the person making the request is still on active duty, or in the National Guard, reserves, etc. Any person who has served in the military can have their records corrected.
The Board for Correction of Military Records can also upgrade discharges requested after the 15 year period for the Discharge Review Boards, and acts as the appellate authority for the Discharge Review Board. Boards for Correction of Military Records can upgrade discharges resulting from a court-martial, although such review is limited to matters of clemency.
Boards for Correction of Military (or Naval) Records have much broader power than Discharge Review Boards or even Federal Courts. Any adverse military record can be corrected, provided that the record is determined to be "erroneous" or "unjust."