[Sony Xperia Z Ultra] blueether:
This is a guide has two functions:

Downgrading firmware to be able to root using Bin4ry's "Root MANY ANDROID" tool or [NUT]'s XZDualRecovery Installer

Restoring your imei if you downgrade from 4.3 or 4.4 to a 4.2 based ROM or FTF (Sony FTF or custom ROM)

Symptoms of lost imei are non working cell reception, no data and the like. If you go to "settings / about phone / status" you will see that the imei is either 0 or many 0's. This happens becuase of imcompatable baseband/kernel.

Prerequisite Steps:

Make sure you have a working backup of your data.

Charge phone

Download latest version of flashtool (link below)

Lost your IMEI

Follow these steps first:

Re-install the firmware that you came from and make sure IMEI is ok

or

come from a clean Stock Sony 4.3:

Come from a stock .290 or .136 so download that for your phone model if needed

Flash .290 or .136 all defaults if not coming from stock

Then follow the Downgrade path below, you can then flash any firmware or ROM before 14.2.A.0.290 / 4.3 without issue.

If you want to latter run a 4.3 or 4.4 based ROM you will have to flash atleast a 4.3 FTF so 14.2.A.0.290 or 14.2.A.1.136

Downgrade to gain root

Follow one of the Downgrade paths below to end up with a 14.1.B.1.532 firmware that is rootable while keeping your imei # intact and working cell/data

Downgrade path 1 (very easy):

Download needed files

Download C6833_14.1.B.1.532_NUT.ftf

Flashing using flashtool

Flash EVERYTHING in the above ftf by [MENTION=4904057][NUT][/MENTION]

Reboot

Flash EVERYTHING in this ftf, (use the defaults) it will do the downgrade below so you will end up on .532 with a working IMEI ready to root with bin4ry's tool

http://xdaforums.com/devdb/project/dl/?id=3442

You will end up with a device that thinks it is a C6833 but when you flash 4.3 or newer (.290 or .136) just use the one for your real model and the model number will be restored.

Downgrade path 2 (easy):

Download needed files

Download a suitable Sony firmware for your model from the ftf thread [see below] - .532

Download a suitable Sony firmware for your model from the ftf thread [see below] - Needs to be .257

It may pay to come from a stock .290 so download that as well

Flashing

Flash .290 all defaults if not coming from stock - just to be safe

Flash .257 Untick Cache, Data and Apslog in the top [BOX]; Exclude system in the second [BOX];

Flash .532 Leave Cache, Data and Apslog ticked] in the top [BOX]; Exclude everything except kernel and system in the second [BOX];

Reboot

Keeping Data

You can untick the data wipe in both of the downgrade paths above if you want to keep user data, but may make it unstable. If you do decide to keep the data partition then wipe dalvik-cache to prevent bootloops and FC's

Thanks to:

[MENTION=4904057][NUT][/MENTION] for new downgrade method and the one step FTF

and these people for the old version of this:

[MENTION=4251096]cgomezparra[/MENTION] for the first fix (now below)

[MENTION=615654]reborn[/MENTION] for first posting/documenting the issue

[MENTION=4742570]abuihsan[/MENTION] for second method: http://xdaforums.com/showpost.php?p=48585720&postcount=6

[MENTION=737884]no name[/MENTION] for #4 http://xdaforums.com/showpost.php?p=49161954&postcount=35

[MENTION=4340984]krabappel2548[/MENTION] for the Flashtool guide

[MENTION=2807379]Androxyde[/MENTION] for Flashtool

others that have tested this

Original thread:

http://xdaforums.com/showthread.php?t=2575359

Links:

FTF files: http://xdaforums.com/showthread.php?t=2453921

Flashtool: http://www.flashtool.net/download.php

Flashtool Guide: http://xdaforums.com/showthread.php?t=2428319

More on Flashtool: http://developer.sonymobile.com/services/flash-tool/

Z-Ultra device files: https://mega.co.nz/#!nQ4GQJpC!UMe9mFfpwzcQ48VSv00cIy3J9nFlzS38UC_iCuXA-3U

Please post any corrections etc