As far as I can see this info has not been pass on from WinCentral. The Upgrade Advisor app, that was used to easily upgrade from the older Windows Phone OS, is no longer available. While it is possible to download the app via the Store, it won’t actually allow the device to be upgraded, instead showing a message that reads: The Windows 10 upgrade isn’t available for your phone right now. We’re trying to determine if your phone can support Windows 10. It makes no difference what phone or if has worked for you before.Only way now is through insiders. Microsoft ends the Upgrade Advisor app, no official upgrade path from Windows Phone 8.1 to Windows 10 Mobile
Moved to Windows 10 Mobile. EDIT: Out-of-date article.