No person shall be appointed a commanding or staff officer unless such person shall have had federally recognized commissioned military service, nor shall any officer be appointed who fails to qualify as to fitness for military service under such regulations as the Secretary of Defense shall prescribe. Such officers may hold their positions until they have reached the age or length of service as prescribed in existing or subsequently amended National Guard regulations, unless separated from the service prior to that time by reason of resignation, disability, withdrawal or termination of federal recognition or commission, or upon finding of a legally convened court-martial.