Federal Sentencing Guidelines "law" or "advice" ex post factoly?

"As constitutional landmark cases go, Peugh v. United States, also heard by the Supreme Court on Tuesday, Feb. 26th., has flown well under the radar thus far," ScotusBlog's Rory Little posted Mon, February 25th., "but at issue is no less than the appropriate constitutional test for identifying an 'ex post facto' law. When the U.S. Sentencing Commission changes its (now advisory) Guidelines to treat certain sentencing factors more harshly, does it violate the Ex Post Facto Clause to apply such changed Guidelines to criminal conduct that was completed before the change?"

Peugh's petition for a writ of certiorari is here; Solicitor General's reply in opposition here.