Whenever an officer who has been selected for promotion by a selective board convened under the provisions of Federal Personnel Act (Sections 3362 and 8366, Title 10, U. S. Code and subsequent legislation) is immediately faced with withdrawal of his federal status as a National Guard officer and transfer to the Army Reserve, he may be transferred or assigned to a suitable position vacancy in which he can be promoted, subject to concurrence of commanders concerned and approval of the Adjutant General.