Allright, I went through the entire process again yesterday. A couple of notes, my 1020 came with the UK O2 firmware installed and I couldn't get the Denim update for some reason (maybe because I live in Germany, the Denim update *should* be available to UK models according to the MS website). Anyhow, I run the Euro DE O2 firmware without any issues on the day to day basis, which does allow me to get all the way up to Denim on 8.1.
So here is what I did yesterday:
- I flashed the Euro DE O2 Postpaid 64GB YLW (059T677) FW and did not restore my backup
- Got a couple of 8.1. updates plus Lumia Denim
- Installed the Windows Insider app and initialized the upgrade
As usual, some Lumia apps were still there after the upgrade process was complete, the Lumia camera app was gone. Also, the Denim firmware update got screwed during the update process, so there was that as well (it does so every time I update to W10).
Next, I ran a hard reset:
- The entire process took roughly about 20 minutes
- Firmware was downgraded to Lumia Cyan
- Lumia Camera was back (Initially it did not open, getting all available updates from the Store fixed that. In my particular case, some dependencies were missing, most notably Visual C++ Redist.)
- Following Lumia / Nokia Apps were brought back after the reset (Lumia Beamer, Lumia Cinemagraph, Lumia Creative Studio, Lumia Help, Nokia Glam Me (Lumia Selfie update errors out, I am pretty sure I was able to update on the UK firmware) plus the usual system apps like Glance and Nokia Account). Everything else is gone (the only thing I personally miss is Lumia Refocus).
So, depending on your firmware, your mileage may vary, but generally, hard resetting your phone after the initial W10 upgrade should bring back the Lumia Camera app. I did not test whenever you can restore your data - it may or may not work. Also, your Denim update might be gone after the update, but I am not sure that it brought any real benefits to the 1020 camera in the first place...
I would love to hear if this approach helps...