Certain downward adjustments to a guideline range, including reduction for time served, are modifications to a certain range; a retroactive modification in that range does not justify a challenge to the sentence where the initial downward reduction resulted in a below-guidelines sentence.
http://media.ca8.uscourts.gov/opndir/18/06/171284P.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.