Best Practices to upgrade to vSphere 5.1 or to vSphere 5.5
1. Upgrade the vCenter Server to 5.5
There are two methods of upgrading your current vCenter to vCenter 5.5:
-The in-place upgrade method: Run vCenter 5.5 installer on the machine having any one from vCenter Server 4.0, vCenter Server 4.1, vCenter Server 5.0, vCenter Server 5.1 having an OS Windows Server 2003 Service Pack 2, Windows Server 2003 R2, Windows 2008 Service Pack 1 and Service Pack 2, Windows 2008 R2, Windows 2008 R2 Service Pack 1.
-Migrating vCenter data using the vSphere Data Migration tool: migrate the current vCenter data, using the vSphere Data Migration tool If your existing vCenter is running on a 32-bit operating system, then you cannot install vCenter 5.1 on the same machine because it is a 64-bit application.
2. Upgrade the ESX/ESXi servers to 5.5
3. Upgrade VMware Tools and then the virtual machine hardware
4. Uninstall vCenter Update Manager, vCenter Converter, vCenter Guided Consolidation and re-install them
vCenter Server 5.5 can be used to manage ESX/ESXi 4.x and ESX 5.0, ESXi 5.1.