What are the implications of Proposition 47's reclassification of theft and drug possession as a misdemeanor?

Enacted in 2014 as the Safe Neighborhoods and School Act, Proposition 47 reclassified several theft and drug possession offenses as misdemeanors (People v. Rivera (2015) 233 Cal.App.4th 1085, 1089), and provided a mechanism to redesignate existing felony convictions for those offenses as misdemeanors ( 1170.18). Specifically, section 1170.18, subdivision (f), provides that a person "who has completed his or her [felony] sentence for a conviction, whether by trial or plea," may be eligible for redesignation if he or she "would have been guilty of a misdemeanor under this act had this act been in effect at the time of the offense."