After downloading the Nokia Software Recovery Tool and suffering through a painfully slow download (even with a 50 Mbits Comcast Business Account), I tried several times to restore with 2 or 3 different failure messages.
Got "Phone cannot be restored" and "Phone flashing has been canceled" and forgot the third message.
Finally got the downgrade to install, then applied the AT&T update and restored the backup.
I followed the various steps from the different threads, but had to do a modified series of steps.
1. Backup the phone to your Microsoft OneDrive account. (Even make a local backup of all the folders you can access from your computer while the phone is connected via USB).
2. Delete the backup from the phone Settings>>Backup>>Apps+Settings and then back it up again to your Microsoft account.
3. Disconnect your phone from the computer.
4. Hard reset the phone (but DO NOT restore nor connect to your Microsoft account).
5 While waiting for the hard reset, uninstall the Nokia Software Recovery Tool (and other Nokia software) from your computer. After it is uninstalled, delete the Nokia folder under the ProgramData folder. (I also uninstalled the Windows Phone software from the same computer). REBOOT.
6. Once the phone is done from the hard reset (DO NOT connect it to your Microsoft account or restore the back), power it down and remove both the SIM and the SD Card.
7. Before re-installing the Nokia software, power the phone on and connect it to the computer. If the computer asks what to do when it sees the phone, select "Do Nothing".
8. Disconnect the phone and install the Nokia Software (right click on the install file and select 'Run as Administrator').
9. Once the Recovery Tool is installed, run it and follow the prompts.
10. If the install succeeds, it will reboot the phone. You will get a 'SIM ERROR', because neither the SIM card nor the SD Card is installed.
11. Go through the setup questions, but do not connect to your Microsoft account or restore.
12. Power down, put the SIM card in and power it up.
13. Go through the 2 to 4 updates from the Settings>>Phone Updates (including all the reboots).
14. When the Phone Updates say there are no more updates available, power down and install your SD Card.
15. Power up, answer any questions and then do a hard reset.
16. During the install setup, make sure to connect to your WiFi and sign in to your Microsoft account. Select restore your data.
Make sure to have a full battery and only during the final restore keep it plugged in.
This is from someone who has a 521, 630, 920, 925 and 1520 (plus a 2520).