VirtualBox: After upgrading guest Windows OS, reinstall GuestAdditions

Title says it all really.  If you are running a guest instance of Windows 10 with GuestAdditions installed in the guest and then upgrade the guest Windows OS, you must re-install GuestAdditions.
If you do not re-install GuestAdditions then features such as Shared Folders will no longer work - they may work and then fail or simply no longer work.