[Q&A] Downgrade 4.4.4 to 4.3 and sim unlock i9505 topic

Dear fellas,

A friend of mine gave me yesterday his phone, asking if I could unlock his carrier lock.
I found several tutorials where this can be achieved through *#0011# menu, but unfortunately the phone currently runs CM11 4.4.4 and that menu does not work.

Can anyone please help me understand what is the correct procedure and files I need to downgrade to stock 4.3 and unlock the sim?
I am familiar with Odin and flashing/back-up procedures, but the very big amount of bootloader/kernel/roms variants, un-restorable bootloaders and stuff just make me more confused.
I am trying not to brick the device, downgrading has always been a weird thing to do.

I found this thread but I am not sure which I should use for a proper process.

Model: GT-i9505
Root: Yes
Android: 4.4.4 CM11
Baseband: I9505XXUGNI2
Kernel: 3.4.104-cyanogenmod-gf25e918 build03@cyanogenmod #1
Build number: cm_jflte-userdebug 4.4.4 KTU84Q

Thanks in advance!