vSphere 5.5 is the oldest supported version of an upgrade to vSphere 6.5. Before we begin, let's review vSphere 5.5 component architecture so that we have a clear understanding of what needs to be upgraded. vSphere 5.5 had separate components.

If you have environments running versions older than vSphere 5.5, you will either need to update the components to vSphere 5.5 first or perform a fresh installation of vSphere 6.5 and then move the workloads to the new environment. In such cases, it is quite possible that the older hardware is no longer supported to host vSphere 6.5 or its components. Use the steps provided in the Planning vSphere upgrade section to review your current environment.

In this section, we will cover the steps involved in upgrading a vSphere 5.5 environment to vSphere 6.5:

1. Backup the current configuration: Take snapshots of SSO, vCenter, and database VM before you start the upgrade. Also, take backups of the database if vCenter is running on a physical machine and using an external database.
2. Upgrade SSO servers to vSphere 6.5 PSC: Regardless of the platform (Windows or vCSA), the Single Sign-On component servers should be upgraded from 5.5/6.0 to vSphere 6.5 before the vCenter upgrade.
3. Upgrade vCenter to VCSA 6.5: For instructions on how to migrate from Windows to VCSA 6.5, read the section Upgrading vCenter Server - Migrating from Microsoft Windows to VCSA of this chapter. Single Sign-On and other services will be migrated. vCenter 6.5 can also be installed on a Windows Server, so upgrading vCenter can also be performed without having to rebuild a new machine. Read the section Upgrading vCenter Server on Microsoft Windows for instructions. In either case, a database upgrade will be performed.

4. Upgrade vSphere Update Manager: VUM will be upgraded and made part of the vCenter Server if the current vCenter system being upgraded also has VUM installed on it. If VUM is installed on a separate machine, which is mostly the case in enterprise infrastructures, then you will need to run the vCenter Migration Assistant on the VUM machine as well.
5. Use vSphere Update Manager to upgrade the hosts to ESXi 6.5: Read the Chapter 14, Upgrading and Patching using vSphere Update Manager for instructions on how to use VUM to upgrade ESXi hosts by scheduling upgrades/updates.
6. Use vSphere Update Manager to upgrade the virtual machine hardware and VMware tools: Read Chapter 14, Upgrading and Patching using vSphere Update Manager, for instructions.

When you upgrade from vSphere 5.5 to vSphere 6.5, you start with upgrading all the SSO instances. When the SSO instances are upgraded, the existing vSphere 5.5 environment will remain unaffected and will also be accessible via the already existing instance of the vSphere Web Client. If the existing SSO is not embedded, then the upgrade will result in a separate vSphere 6.5 PSC instance. The result remains the same regardless of the platform vCenter is deployed on. If you have more than one vCenter server connecting to the same SSO domain, then post the upgrade of one of the vCenter Servers, the newer vSphere Web Client 6.5 can be used to view/manage both vSphere 6.5 and 5.5 vCenter Servers. If you have more than one SSO/PSC servers, then upgrading one among them will not affect any of the services including vCenter Servers, except for the linked mode configuration, which will not be able to link two disparate vCenter versions.

Another important aspect to keep in mind is that the Platform Services Controller (PSC) and vCenter Server (Appliance or Windows) will manage two separate sets of services. The following table lists some of the services managed by both the components: