(a) If a regional planning commission prepares and adopts a regional plan, the regional plan or a portion thereof may then be adopted by the legislative body of any member municipality as its plan in accordance with subchapter 5 of this chapter.
(b) The legislative body of any municipality may designate the regional planning commission of a region of which such municipality is a member as the planning commission of such municipality, and, if so designated, the regional planning commission shall thereafter act as the planning commission of such municipality until a planning commission is created under section 4321 of this title or until such regional planning commission notifies such legislative body, in writing, that it no longer will so act.