Freesurfer v5

I have a sample of subjects processed with v5. 0 of freesurfer for a volumetric study. Unfortunately, I don't have the DICOM files of those subjects anymore. I only have the .mgz files that resulted from processing the images using freesurfer (v5.0). Nevertheless, I would like to use those subjects and be able to reprocess them using the new version of freesurfer, without incurring any bias. So I would like to get the original nifi and/or dicom file (from the .mgz) so I can reprocess them . Any other solution that works and don't causes biases from using different versions of freesurfer will be accepted.