First, my phone has stock rom, v. 12.1.A.1.201 with JB 4.3. And I just want to flash a stock rom (maybe a little tweak to make it smooth and faster) + root.
So I searched (maybe not too good) how to do it, and I found this: to install any rom, you need to have an unlocked bootloader, so far so good, but then when you search on how to unlock it, surprise! you need to be root to backup drm keys ... Ok then, I tried another aproach, maybe flashing from custom recovery, but then again to install a custom recovery you need to be rooted ... back to the begining.
So if anyone can enlighten on how to fulfill my very reasonable request, I would be sooo happy.