ons 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.3d 177, 189 (4th Cir. 1993). See also PradoSteiman ex rel. Prado v. Bush, 221 F.3d 1266, 1273 (11th Cir. 2000). The distr