Where an attorney-general instructs that a trial be converted into a preparatory examination, it shall not be necessary for the magistrate or regional magistrate to recall any witness who has already given evidence at the trial, but the record of the evidence thus given, certified as correct by the magistrate or regional magistrate, as the case may be, or, if such evidence was recorded in shorthand or by mechanical means, any document purporting to the transcription of the original record of such evidence and purporting to be certified as correct under the hand of the person who transcribed it, shall have the same legal force and effect and shall be admissible in evidence in the same circumstances as the evidence given in the course of a preparatory examination: Provided that if it appears to the magistrate or regional magistrate concerned that it may be in the interests of justice to have a witness already examined recalled for further examination, then such witness shall be recalled and further examined and the evidence given by him shall be recorded in the same manner as other evidence given at a preparatory examination.