S. App. LEXIS 7739, ea; 126 Fair Empl. Prac. Cas. (BNA) 1793; 99 Empl. Prac. Dec. (CCH) P45,306 54(b). "[C]ertifications are not frozen once made," Amgen, Inc., 133 S. Ct. at 1202 n.9, and a district court has "considerable discretion to decertify the class," Cent. Wesleyan Coll. v. W.R. Grace & Co., 6 F.3