What is the legal test for petitioning to change a criminal conviction to a misdemeanor?

In November 2014, the California voters approved Proposition 47, an initiative that reclassified as misdemeanors certain nonviolent drug and theft offenses previously cast as felonies. (People v. Valenzuela (2019) 7 Cal.5th 415, 418.) Proposition 47 also created a petitioning procedure making those changes available to offenders who had previously been convicted of reclassified offenses. ( 1170.18.) To obtain relief under this procedure, the petitioner must demonstrate they "would have been guilty of a misdemeanor . . . had [Proposition 47] been in effect at the time of the offense." ( 1170.18, subd. (a).)