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Upgrade process considerations 
You need to be aware of several considerations before you upgrade the configuration version of 
a VM: 

You must shut down the VM before you upgrade the VM configuration version. 

The configuration version upgrade process is one way; that is, when you upgrade the 
configuration version of the VM from version 5.0 to version 6.0, you cannot downgrade, and, 
hence, afterward you cannot move the VM to a server running Windows Server 2012 R2. 

The Update-VMConfigurationVersion cmdlet is blocked on a Hyper-V cluster when the cluster 
functional level is Windows Server 2012 R2. You can still move the VM between all of the nodes in 
the Hyper-V cluster, however, when the cluster has a mix of both Windows Server 2012 R2 and 
Windows Server 2016. 
More info To read more about the upgrade process, go to 
https://technet.microsoft.com/en-
us/windows-server-docs/compute/hyper-v/deploy/upgrade-virtual-machine-version-in-hyper-v-
on-windows-or-windows-server
. 

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CHAPTER 2 | Software-defined datacenter 
New configuration file format 
After you have upgraded the VM configuration version as described in the previous section, the VM 
will use the new configuration file format. The new VM configuration file format uses the .vmcx 
extension for the VM's configuration data and the .vmrs extension for its runtime state data. The 
new format is a binary file format, which means that you cannot edit the file directly. The new 
configuration file format increases the efficiency of reading and writing the VM's configuration data, 
reduces the potential for data corruption in the event of a storage failure, and provides better overall 
efficiency. 
Figure 2-16 shows the new VM configuration file format, which uses the .vmcx extension for the VM’s 
configuration data and the .vmrs extension for runtime state data.