OK - mine worked on Win7 with Nokia Software Recovery tool on AT&T Lumia 1520, but ...
- I repeat, I downloaded the latest version of NSR tool
- BEFORE backing up (the phone).
-- I pulled the SDcard to maintain my pix, vids, docs & music.
-- I moved all the apps installed on the SDcard to the phone.
- took a FULL backup of the phone (Settings >> Backup)
- took forever for the phone to be recognized by NSR tool (~ 10 minutes)
- downloading the software(ROM), to downgrade the phone to WP 8.0 took forever
- after that, it went relatively smoothly
- once I was on WP 8.0, I rebooted (again)
- after that (phone update), it installed a small setup for Cyan, then Cyan
- restoring the backup, was a bit problematic the first time
- did a HARD RESET, then immediately signed in to my MS account
- after that, the backup did it's thing (which you guessed it, took forever)
Took the better part of 5 hours to get 90% back (went from WP 8.1 Dev Preview, to WP 8.0, to Lumia Cyan w WP 8.1, then restoring the backup). Got to 100% earlier this afternoon, data restored from laptop (Easy Transfer works over WiFi network). Total time less than 24 hours & I was in NO hurry to get from 90 to 100%.
BTW - if you got the unlock code from AT&T, you're gonna have to unlock, again (use ##7820#).