The license server can work across many version of Citrix and many products. Later products will usually require the latest versions of the License Server. This means that if you currently have a license server in place and you add a newer product such as XenApp 6.5 or XenServer 6 you will need to upgrade your current server. This is easy on the Windows version. The MSI will detect the License Server is installed and offer the upgrade. All licenses and configuration is maintained.
Additionally you may want to upgrade to make use of new features that become avaiable such as with the latest version 11.10 on Windows supporting AD integration if the License Server OS is a member of a Domain. This means that you can add in License Administrators and groups from the Domain.