Today morning I noticed a message saying welcome to Lumia Denim on my 1020 as soon as I turned on the phone. And out of joy, I checked the extras+info, and I saw the Lumia Denim software release notice to confirm it. Of course in my settings I set the phone for automatic update and install at 3.00AM as preferred time. We have another 1020 at home and I quickly checked that I didn’t see anything there. The extras+info still shows it as Lumia Cyan. The only difference between these two phones is, the one that got Lumia Denim is developer unlocked and “Preview for Developers” installed. AFAIK, that shouldn’t make any difference because developer unlocked allows you to test your development on the device using Visual Studio debugger and Preview for Developers lets the beta bytes rolling from Microsoft for OS not the firmware. Lumia Denim is part of GDR and needs to be tested by AT&T or whatever carrier you use to roll onto your phones.
The about settings shows different versions for both the Lumia 1020s definitely. The upgraded phone with Lumia Denim shows higher version numbers for OS. You can notice that in the following screenshots of the phones.
Note: I also happen to developer unlocked and preview for developers installed Lumia 920, but that didn’t get the Denim either.