An upgrade of vCenter Server can be done either using the vCenter 5.5 Simple Installer or by initiating the vCenter Server's installer separately (recommended). It can be performed only if the previous version of vCenter is on a supported 64-bit Windows operating system.
The following Windows operating systems are supported by VCenter:
Starting with vCenter 5.5, Windows 2003 is no longer supported.
Only the following releases, if installed on any of the preceding operating systems, are eligible for an in-place upgrade: