Which change to a running Azure virtual machine will cause downtime?

Changing the size of a running Azure virtual machine to a different SKU, such as D8s v3, will cause downtime because this operation requires the virtual machine to stop and be deallocated. During this process, resources are released and then reallocated to accommodate the new size specifications. This deallocation and subsequent reallocation lead to a temporary unavailability of the virtual machine, meaning that any services or applications running on it will be interrupted.

In contrast, adding a managed disk, attaching an additional network interface, or installing an extension—like the Puppet Agent—can typically be done without stopping the virtual machine. These operations are designed to be non-disruptive, allowing for changes to be made while the VM remains operational. Therefore, modifying the virtual machine size stands out as the action most likely to cause downtime.