Assign 2 GB of RAM to Hyper-V machines before trying to upgrade Windows 10

Hyper-V has a dynamic memory feature that allows it to change how much RAM is allocated to the virtual machine while it's running. The absolute minimum - usually much less than will be allocated for realistic workloads - is set in the VM configuration. When starting the upgrade to a new build of Windows 10, the setup infrastructure checks whether the machine has at least 2 GB of RAM. If the check fails, it cancels the upgrade and deletes the update files. To avoid needing to wait for the update to be downloaded again, I'd advise making sure the VM is guaranteed 2 GB before trying to start the upgrade.