When Microsoft launched Windows 10 Mobile for older Windows Phones, the company had also launched an app “Upgrade Advisor”. The app lets you upgrade from Windows Phone 8.1 to Windows 10 Mobile and it is the only official upgrade path.
In the past, Microsoft has blocked the ability to upgrade from Windows Phone 8.1 to Windows 10 Mobile, at that time the issue was assumed to be a backend bug as it was later fixed. Today, Microsoft has once again made it impossible for the old Windows Phone 8.1 users to upgrade to the new Windows 10 Mobile operating system.
As first spotted by a German website, the Upgrade Advisor app for Windows Phone is not available for download anymore. It means if you cannot install the update, it wouldn’t be possible for you to officially upgrade the old phones to the operating system.
It could be possible that the removal of Upgrade Advisor has something to do with the Windows 10 Mobile Fall Creators Update (feature2) release. Microsoft might release the next update for Windows 10 Mobile after releasing the update for PCs in October.