Just found this on the MS Windows 10 Specifications page:

~Applications, files and settings will migrate as part of the upgrade, however some applications or settings may not migrate.
  • For Anti-virus and Anti-malware applications, during upgrade Windows will check to see if your Anti-virus or Anti-malware subscription is current. Windows will uninstall your application while preserving your settings. After upgrade is complete, Windows will install the latest version available with the settings that were set prior to upgrade. If your subscription is not current, upgrade will enable Windows defender.
  • Some applications that came from your OEM may be removed prior to upgrade.
  • For certain third party applications, the “Get Windows 10” app will scan for application compatibility. If there is a known issue that will prevent the upgrade, you will be notified of the list of applications with known issues. You can choose to accept and the applications will be removed from the system prior to upgrade. Please be sure to copy the list before you accept the removal of the application.

Useful to know that there might be a problem with AV during the upgrade and that OEM applications may be removed.

This sounds more like a Restore or Refresh action than the W8 to W8.1 upgrade. I wonder how many people will just hit the upgrade button without being aware of this though.

I hope my AV makes it through the upgrade having just got it working again after some issues.

I'm looking forward to having a new copy of Microsoft Solitaire though and really hoping that there will be a way to uninstall Candy Crush Saga and not just hide it.