VMware Licensing Changes when you Upgrade ESXi 5.1 to 5.5
As per VMware KB the vSphere 5.5 also do not need any new licenses the existing 5.0 or 5.1 licenses willwork and enable all the features of the software once upgraded. The ESXi 5.1/5.5 features are automatically enabled after you upgrade from ESXi 5.0 to 5.1/5.5. This does not need a new license key.
I have checked my old ESXi 5.1 license with ESXi 5.5 it works without any issue. So there is no need to upgrade ESXi license if you want to upgrade from 5.0 or 5.1 to ESXi 5.5.
All ESXi 5.x and vCenter Server 5.x keys work with ESXi 5.1/5.5 and vCenter Server 5.1/5.5. To update your software from 5.0 to 5.1/ 5.5, see