I am just writing these steps to help those who want to downgrade from KK to JB due to any reason. Please use these steps at your own risk.
Recently I updated my Razr M to KK, it was Locked and Unrooted, but with the help of Twelroot and Motopocalypse I rooted and unlocked the BL.
Now somehow I didn't like the KK and wanted to go back to JB 4.1.2 because I was now having unlocked BL and rooted phone so it was easy but need to follow some steps to successfully downgrade to JB. So an unlocked BL and rooted with KK is must.
USB drivers for Razr M must be installed before proceeding.
1. download the firmware file [Android 4.1.2
Blur_Version.98.30.1.XT907.Verizon.en.US] from http://sbf.droid-developers.org/phone.php?device=8
2. extract the above archive and replace the XML file with the one in attachment.
3. put the phone into fastboot mode and flash the firmware with RSD Lite (attached)
4. reboot and now you have JB 4.1.2
If you want to install root application again and also want to flash custom recovery then please follow the Post#5 [ATTACK] (thanks ATTACK)
Note : for custom recovery you will need the fastboot.exe if you don't have it then install minimal_adb_fastboot_v1.1.3_setup.exe(attached).
Put the phone in fastboot mode, copy the recovery to c:\ and execute the below command :
fastboot flash recovery [Path to recovery image]
If it help you then don't forget to press Thanks button !