VMware supports in-place upgrades on 64-bit systems from vCenter Server 4.x, vCenter Server 5.0.x, and vCenter Server 5.1.x to vCenter Server 5.5.
Unlike versions before vCenter Server 5.1, vCenter Server 5.5 does not support directly migrating an existing, 5.0.x or earlier vCenter Server to a new machine during an upgrade to version 5.5. You can migrate such an existing vCenter Server to a new machine during an upgrade to version 5.0.x, and then perform an in-place upgrade from version 5.0.x to version 5.5. See Upgrading to vCenter Server on a Different Machine.
Previous versions of vCenter Server use a 512-bit key self-signed certificate, however vCenter Server 5.5 requires a 2048-bit key certificate. Certificates are not replaced during upgrade. For a successful upgrade, replace the 512-bit key certificate with a 2048-bit key certificate using one of the following options.
You can perform a fresh installation of vCenter Server 5.5.
For vCenter Server 5.1, you can use the steps in Knowledge Base article 1014953 to replace SSL certificates.
For vCenter Server 5.0, you can use the steps in Knowledge Base article 2015421 to replace SSL certificates.
vCenter Server 5.5 can manage ESX 4.x/ESXi 4.x, ESXi 5.0.x, and 5.1 x hosts in the same cluster with ESXi 5.5 hosts. vCenter Server 5.5 cannot manage ESX 2.x or 3.x hosts.
You cannot upgrade a vCenter Server 4.x instance that is running on Windows XP Professional x64 Edition to vCenter Server 5.5, because vCenter Server 5.5 does not support Windows XP Professional x64.
vSphere 5.1 introduced vCenter Single Sign On service as part of the vCenter Server management infrastructure. This change affects vCenter Server installation, upgrading, and operation. See How vCenter Single Sign-On Affects vCenter Server Upgrades.