I have successfully updated a Nokia 521 and 925 to WP 8.1 but the 810 is a frikkin pain in the rear. It gives me all BS reasons and I have used Nokia Software recovery tool to restore the phone and try everything again like 5 times already. Once I was able to get it to the point where it said you'll now be able to enjoy 8.1 but from that point onward, it's been nothing.
I am trying to move from Android to Windows and this is not fun. Why can't you sideload 8.1 like I can do with custom roms through recovery on Android?