Your initial vCenter Server 5.5 configuration determines your upgrade and configuration options.
The vCenter Server 5.5 example upgrade paths demonstrate some common starting configurations before vCenter Server upgrade and their expected configuration outcomes after vCenter Server upgrade.
If you have a simple installation with all vCenter Server 5.5 components on the same system, the vCenter Server 6.5 software upgrades your system to vCenter Server with an embedded Platform Services Controller instance. The software upgrades your vCenter Server common services such as vCenter Single Single-On in the Platform Services Controller instance. The rest of the vCenter Server components, such as vSphere Web Client Inventory Service, are upgraded to 6.5 as part of the vCenter Server group of services. The software upgrades vCenter Server and all its services in the correct order to the same version.
If you have a custom vCenter Server 5.5 environment with an externally deployed vCenter Single Sign-On, the vCenter Server 6.5 software upgrades your deployment to vCenter Server with an external Platform Services Controller instance.
If your configuration includes a vSphere Auto Deploy server, the upgrade process upgrades it when upgrading the associated vCenter Server instance. You cannot use a vSphere Auto Deploy server that was included with an earlier version of the product in conjunction with vCenter Server 6.5. If your vSphere Auto Deploy server is running on a remote system, it is upgraded and migrated to the same system as vCenter Server during the upgrade process.
For example, if your vCenter Server is part of vCenter Server Appliance, and you installed the vSphere Auto Deploy server on a Windows machine, the upgrade process migrates the vSphere Auto Deploy server to the same location as your vCenter Server Appliance. Any settings are migrated to the new location. However, you must reconfigure your ESXi hosts to point to the new vSphere Auto Deploy location.
If your configuration includes a remotely deployed vSphere Web Client, it is upgraded along with the vCenter Server instance to which it is registered and migrated to the same location as the vCenter Server instance.
Only the vCenter Single Sign-On instance remains remotely deployed as part of the the Platform Services Controller instance after upgrade to vCenter Server 6.5. If all vCenter Server components are deployed remotely, all are migrated to the vCenter Server location during the upgrade except vCenter Single Sign-On. While Inventory Service data is migrated to the vCenter Server location, the legacy version is no longer used and must be uninstalled manually. See Distributed vCenter Server 5.5 for Windows Services Relocation During Upgrade or Migration
If you have multiple systems configured for high availability, vCenter Server enables you to incorporate your common services into an external Platform Services Controller configuration as part of your upgrade process.
If you have a multisite setup configured with replication, you can use vCenter Server to incorporate your common services into an external Platform Services Controller configuration as part of your upgrade process.
For more information on mixed version transitional environments, see Upgrade or Migration Order and Mixed-Version Transitional Behavior for Multiple vCenter Server Instance Deployments.