Within VMware Cloud Director 10.6.1 and earlier there is no supported method to alter a named (shared) disk to an independent disk.
VMware Cloud Director 10.6.1
There is only a workaround that is recommended. The VM with the attached named disk should be downloaded following https://techdocs.broadcom.com/us/en/vmware-cis/cloud-director/vmware-cloud-director/10-6/map-for-vmware-cloud-director-tenant-portal-guide-10-6/working-with-vapps-tenant/download-a-vapp-as-an-ova-tenant.html. Then the vApp can be re-deployed, and the disk will be dependent (https://techdocs.broadcom.com/us/en/vmware-cis/cloud-director/vmware-cloud-director/10-6/map-for-vmware-cloud-director-tenant-portal-guide-10-6/working-with-vapps-tenant/create-a-vapp-from-an-ovf-package-tenant.html).