If a petition filed in the juvenile court alleging that a minor comes within the provisions of Section 602 alleges that a minor has committed an offense that would, in the case of an adult, be punishable alternatively as a felony or a misdemeanor, the court, subject to a hearing, at any stage of a proceeding under Section 602, may determine that the offense is a misdemeanor, in which event the case shall proceed as if the minor had been brought before the court on a misdemeanor petition.