Upgrading the ESXi or vCenter Server software may require changes in the licensing configuration of ESXi hosts or vCenter Server.
When you apply a minor upgrade or patch to the ESXi or vCenter Server software, you do not need to replace the existing license key with a new one.
If you apply a major upgrade to ESXi 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.x license.
Similarly, if you upgrade vCenter Server from 4.x to 5.x, you must assign a vCenter Server 5.x 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.