When the legislative branch changes the rules, it is helpful if it specifies the extent of retroactive effect.  However, those decisions can get intricate, and adjustment might be needed if the Supreme Court alters its Ex Post Facto Clause jurisprudence yet again.  The legislature might prefer to delegate the issue to the executive.  Is that constitutionally permissible?

In 2010, the Second Circuit decided United States v. Guzman, 591 F.3d 83 (emphasis added):

The provisions of SORNA relevant to this appeal are fairly straightforward. Under 426 U.S.C. § 16913(a), a convicted sex offender must register "and keep the registration current, in each jurisdiction where the offender resides, ... is ... employ[ed], [or] is a student." With respect to initial registration, a sex offender must also register in the jurisdiction of conviction if it is not the offender's jurisdiction of residence. Id. For initial registration purposes, § 16913(b) provides that the individual must register "before completing a sentence of imprisonment with respect to the offense giving rise to the registration requirement" or "not later than 3 business days after being sentenced for that offense, if the sex offender is not sentenced to a term of imprisonment." Section 16913(c) requires the registrant to inform at least one of the relevant jurisdictions of all relevant updates to his or her registration information within three days of a "change of name, residence, employment, or student status." The jurisdiction so informed must then share that information with other jurisdictions where the offender is required to register. Id.; see also id. § 16921(b)(3). Section 16913(d) gives the United States Attorney General "the authority to specify the applicability of the requirements of this subchapter to sex offenders convicted before [SORNA's enactment on July 27, 2006] or its implementation in a particular jurisdiction, and to prescribe rules for the registration of any such sex offenders and for other categories of sex offenders who are unable to comply with subsection (b) of this section."

The Second said that was kosher and simply cited Guzman in a footnote in the case taken up today.  SCOTUS took the retroactivity delegation question while turning down the other three questions in Guzman v. United States, No. 17-6086.  Amy Howe has this post at Howe on the Court.