We are running VMs of version 2012 R2 on a server licensed with Windows Server 2016 Datacenter. The server itself is only accessed for administrative purposes. The VMs are accessed by different users who represent affiliate companies. One user wants to upgrade his area to version 2016 and thus uses Windows Server CALs 2016. All the other users continue using CALs 2012. Do these users need to upgrade their CALs to 2016?

If the provider and the companies (users) can be considered as affiliate companies, the users can access the server with their CALs. If the physical OSE is only used for hosting and administrative purposes, no CALs are required here. Users accessing VMs that run version 2012 R2 are required to have CALs of version 2012. If there are also VMs that run version 2016, then all users accessing these VMs are required to have Windows Server CALs 2016. In summary, users are not forced to upgrad to version 2016 if they are still accessing Windows Server 2012 R2.

Source: Microsoft Product Terms_June 2017