vCenter Server systems are licensed with vCenter Server licenses that have per-instance capacity.
To license a vCenter Server system, you need a vCenter Server license that has the capacity for at least one instance.
When you install a vCenter Server system, it is in evaluation mode. An evaluation mode license of a vCenter Server system expires 60 days after the product is installed no matter whether you assign a license to vCenter Server or not. You can set vCenter Server back to evaluation mode only within 60 days after its installation.
For example, suppose that you install a vCenter Server system and use it in evaluation mode for 20 days and assign the system an appropriate license. The evaluation mode license of vCenter Server will expire after the remaining 40 days of the evaluation period.
License and Evaluation Period Expiry
When the license or evaluation period of a vCenter Server system expires, all hosts disconnect from that vCenter Server system. If after the license or evaluation period of vCenter Server expires that vCenter Server is assigned to a new license key, all disconnected hosts reconnect to the vCenter Server system.
Licensing vCenter Server After Upgrade
If you upgrade vCenter Server to a version that starts with the same number, you can keep the same license. For example, if you upgrade a vCenter Server system from vCenter Server 5.1 to 5.5., you can keep the same license on the system.
If you upgrade vCenter Server to a major version that starts with a different number, the evaluation period restarts and you must assign a new license. For example, if you upgrade a vCenter Server system from 5.x to 6.x, you must license the system with a vCenter Server 6 license.
If you upgrade the edition of the license, for example, from vCenter Server Foundation to vCenter Server Standard, replace the existing license on the system with the upgraded license.