Mods please sticky this. A lot of people are wasting their time trying different methods that don't work. You can't downgrade fully, you need the cellular modem files from 1.2 for GSM to work.
Use this at your own risk as this creates a "frankenbuild". You will still need the leaked 1.2 image file for future flashing if the cellular modem files aren't updated in the final version of PR1.2.
1. Flash back to 1.1.1 (Use your country specific firmware)
Code:
flasher-3.5 -F RX-51_2009SE_3.2010.02-8_PR_COMBINED_MR0_ARM.bin -f -R
2. Run the following command and replace RX-51_blahblahblah_ARM.bin with the 1.2 image name.
Code:
flasher-3.5 -F RX-51_blahblahblah_ARM.bin --flash-only cmt-mcusw -f -R