You can migrate a vCenter Server instance with an embedded vCenter Single Sign-On (version 5.5) or Platform Services Controller (version 6.0) to a vCenter Server Appliance 6.5 instance with an embedded Platform Services Controller appliance. In this case the software migrates the vCenter Server instance and the embedded vCenter Single Sign-On instance or Platform Services Controller instance at the same time.
For more information on mixed version transitional environments, see Upgrade or Migration Order and Mixed-Version Transitional Behavior for Multiple vCenter Server Instance Deployments
For more details:
vCenter Install/Upgrade/Migrate 6.x (Both ESXi and vCenter Server): https://docs.vmware.com/en/VMware-vSphere/6.5/vsphere-esxi-vcenter-server-65-upgrade-guide.pdf
vSphereCentral: https://vspherecentral.vmware.com/
FeatureWalkthrough: https://featurewalkthrough.vmware.com/