There are three possible outcomes that a Windows Phone 8.1 user can receive from the app. You can be told that your handset is ready to receive the Windows 10 Mobile update, find out that another update must be installed first, or find out that your phone cannot be updated at all to Windows 10 Mobile. The latter is the harshest of the three possibilities.
If Upgrade Advisor tells the Windows Phone 8.1 owner that more room needs to be made on the phone for the update, it will suggest that certain files (like videos and pictures) be stored in the cloud or on a microSD card. The app can make the adjustments right away, even allowing the device owner to change which files are being moved. The app can also help delete unwanted files.
It doesn't appear that Upgrade Advisor is ready yet for prime time, although it does seem like it will be quite useful once released. In the meantime, we can do our part and remind you that the Windows Phone 8.1 models expected to be updated to Windows 10 Mobile include the Microsoft Lumia 430, Microsoft Lumia 435, Microsoft Lumia 532, Microsoft Lumia 535, Microsoft Lumia 540, Microsoft Lumia 635 (1 GB RAM), Microsoft Lumia 640, Microsoft Lumia 640 XL, Nokia Lumia 735, Nokia Lumia 830, and the Nokia Lumia 930.