Please refer section 165 of the Companies Bill. It clearly states that a person holding directorships in excess of 20, within 1 yr of commencement of the Bill has to choose among the directorships, resign in remaining and accordingly intimate his decision.
Therefore, the said section is applicable not only to new appointments but also existing directors.
Further, appended bewow is section 165 (3) of the Comapnies Bill, 2011 for your reference:
(3) Any person holding office as director in companies more than the limits as specified in sub-section (1), immediately before the commencement of this Act shall, within a period of one year from such commencement,—
(a) choose not more than the specified limit of those companies, as companies in which he wishes to continue to hold the office of director;
(b) resign his office as director in the other remaining companies; and
(c) intimate the choice made by him under clause (a), to each of the companies in which he was holding the office of director before such commencement and to the Registrar having jurisdiction in respect of each such company.