Upgrading the ESXi or vCenter Server software might require changes in the licensing configuration of ESXi hosts or vCenter Server.
When you apply a minor upgrade or a patch to an ESXi host or vCenter Server, you do not need to replace the existing license key with a new one. For example, if you upgrade a host from ESXi 5.1 to 5.5, you can keep the same license key on the host. Replace the license key with a new one in case the new version of the host has new features that you want to use.
If you apply a major upgrade to an ESXi host or vCenter Server, you need to apply a new license key. For example, if you upgrade an ESXi host from 4.x to 5.x, you need to license the host with a vSphere 5 license. Similarly, if you upgrade vCenter Server from 4.x to 5.x, you must assign a vCenter Server 5 license.
If you upgrade the edition of the license, for example, from vSphere Standard to vSphere Enterprise, you must replace the existing license key on the assets with the upgraded license key.