On my 920 it didn't overwrite my backups. I originally thought it did.I took someone's advice of doing a factory reset, downloading and updating 10, doing another factory reset, then restoring backup. During one of factory reset on 10, it appeared my 8.1 backup was going. It was still showing my 520 backup, but it wasn't showing my 920 backup. It just showed Windows Phone back. Which was my 10 backup. But after I rolled back to 8.1, I was able to do a restore just like the phone was when I backed up. Of course minus some photos, all music, and all app data; but I'll complain about that another time