As of April 6, 2015, the provisions of subpart AA shall replace the provisions of subpart X, Sec. Sec. 51.900 to 51.918, which will cease to apply, with the exception of the attainment date extension provisions of Sec. 51.907 for the anti-backsliding purposes of Sec. 51.1105(d)(2). [80 FR 12312, Mar. 6, 2015]