18 USC 3582(c)(1)(A)

(c) Modification of an Imposed Term of Imprisonment.-The court may not modify a term of imprisonment once it has been imposed except that-

(1) in any case-

(A) the court, upon motion of the Director of the Bureau of Prisons, or upon motion of the defendant after the defendant has fully exhausted all administrative rights to appeal a failure of the Bureau of Prisons to bring a motion on the defendant’s behalf or the lapse of 30 days from the receipt of such a request by the warden of the defendant’s facility, whichever is earlier, may reduce the term of imprisonment (and may impose a term of probation or supervised release with or without conditions that does not exceed the unserved portion of the original term of imprisonment), after considering the factors set forth in section 3553(a) to the extent that they are applicable, if it finds that-

(i) extraordinary and compelling reasons warrant such a reduction; or

(ii) the defendant is at least 70 years of age, has served at least 30 years in prison, pursuant to a sentence imposed under section 3559(c), for the offense or offenses for which the defendant is currently imprisoned, and a determination has been made by the Director of the Bureau of Prisons that the defendant is not a danger to the safety of any other person or the community, as provided under section 3142(g);

and that such a reduction is consistent with applicable policy statements issued by the Sentencing Commission