I'll try to write a simple guide and script to simplify it.
This is only for testing, always back-up your contacts, photos etc... .
Download files:http://www.mediafire.com/download/p6rt9 ... ngrade.zip
Download files and extract them. Don't edit anything or remove any files, otherwise it wont work.
Desktop seems good place to extract.1)
First install ADB and fastboot driver if not previously installed. Use script nr0 to check ADB or fastboot connectivity(It doesn't indicate missing driver)
If it shows unauthorized, you need to authorize it first.
ADB - Android Debugging Bridge Connectivity to phone when in normal mode and recovery.
Fastboot - Connects to phone when in bootloader. http://forum.xda-developers.com/showthread.php?t=258897
It's also possible to go directly to bootloader. Without enabling USB-debugging.
★Disconnect your USB-cable.
★Power off your phone.
★Boot it in fastboot by pressing and holding Power Button + Volume Down at the same time.
Run 1.Downgrade bootloader.bat script.
It flashes the old radio onto phone and reboots again into bootloader.DON'T PRESS ANY KEY JUST YET!!!
Check if baseband version on phone screen indicates M8974A-22.214.171.124.23
Now you can press any key and your phone boots up.
Congratulation, you have just downgraded your phone Radio(Baseband)
Now you still need to root your phone and install rsap installation files first and then rsap app.
I'm not sure if flashing radio with locked baseband will work, but it doesn't seem to be able to root without modified kernel what still needs unlocked bootloader.