Dear Adair,
additionally to Colleens suggestions you can easily down- and upload mitigations with program GRAC_DOWNLOAD_MIT_ASSIGNMENTS / GRAC_UPLOAD_MIT_ASSIGNMENTS. If you download all mitigations to an Excel file you can easily filter for your monitor who needs to be changed.
Best if you add the new monitor to the control first, then download the mitigation and modify directly in Excel (overwrite the old with the new monitor) and upload again.
Let me know if you need more information in this regard.
Regards,
Alessandro