Warning: This method is not fully tested, may brick your phone! Back up all data, and do not use this method to flash any incompatible ROMs!
- Update your AT&T Lumia 1020 to Windows 10 mobile and interop unlock it.
- Change the registry value of “DataStore” to “C:\EFIESP\efi\Nokia” in “HKLM\SOFTWARE\Microsoft\MTP”.
- Connect the phone to PC, open “Phone\PCONF”, copy “config.pcn” to your HDD and use a HEX editor to open it.
- Change P5217_ATT (HEX: 50 35 32 31 37 5F 41 54 54) to P5217 (HEX: 50 35 32 31 37 00 00 00 00) at offset 0x10, then save it.
- Copy modified “config.pcn” back to “Phone\PCONF” and overwrite it.
- Restart your phone and use flashing tools like “thor2.exe” to flash unbranded RM-875 rom!
- Download the update cab (Lumia 1020 ONLY)
Then copy the cab file to an empty folder, e.g. “c:\cab”.
- Download and install the tools (Thanks WojtasXda!)
- Connect the phone to PC, then run cmd and go to “iutool.exe” folder (default: Program Files (x86)\Windows Phone Kits\8.1\Tools\bin\i386), type
iutool.exe -p c:\cab -V
- Flash unbranded RM-875 rom!
We knew AT&T changed the Platform ID (P5217_ATT) to avoid flashing RM-875 rom (P5217). The flashing tools check the platform info values of the phone and the DevicePlatformID string from the rom. Lumia 635/820 has SD card, and the device info in the registry can be changed. After the phone checking and installing updates, it is unbranded. Thus the updates can change the Platform ID info of the phone. (Thanks pankaj981 for this guide)
I find the update cabs of an unbranded RM-876 phone (flashed RM-875 rom). Then using the iutool.exe to update AT&T RM-877 manually. After that, I flashed RM-877 with RM-875 rom successfully (Method B). Finally, I find “config.pcn” contains Platform ID. Thus for an interop unlocked phone, Method A may be easier.
Post here if you have any questions!