"During September 2017, you can try the new experience by creating new VMs in the US West Central region"Maintenance and updates for Windows VMs in Azure | Microsoft Docs
"Azure periodically performs updates to improve the reliability, performance, and security of the host infrastructure for virtual machines. These updates range from patching software components in the hosting environment (like operating system, hypervisor, and various agents deployed on the host), upgrading networking components, to hardware decommissioning. The majority of these updates are performed without any impact to the hosted virtual machines. However, there are cases where updates do have an impact:
If the maintenance does not require a reboot, Azure uses in-place live migration to pause the VM while the host is updated."
In this case the host OS is being upgraded form Server 2012 (R2) to Server 2016.
New VMs are already being created on Server 2016, now existing long running VMs need to migrate.