When challenging a district court's procedurally correct decision not to lower a sentence imposed with reference to subsequently retroactively amended guidelines ranges, the petitioner must establish that the decision to leave the original sentence in place is itself substantively unreasonable.
https://www.cadc.uscourts.gov/internet/opinions.nsf/2F95E99A58F5CD4D852582AD0054B25F/$file/16-3052-1736019.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.