Within the Manage and maintain Windows Server IaaS virtual machines section, the guide notes operational impacts of common VM changes. It states: "Resizing a virtual machine changes its compute allocation; the VM is restarted and may require stop/deallocate if the target size isn't available on the current host.
Expect downtime during a size change." In contrast, the storage guidance says: "Managed data disks can be attached or removed from a running VM; these operations do not require a VM redeploy and typically do not incur downtime." Adding a new standard SSD (data disk) is supported online, and detaching a data disk can also be performed without shutting down the VM (Windows may require a rescan and clean removal in the OS, but the VM remains running). Therefore, among the proposed changes, the one that requires downtime is changing the VM size to D4s_v4, because the platform must restart (and sometimes deallocate) the VM to re-provision compute resources, whereas attaching/detaching data disks does not inherently require downtime.