[Sony Xperia ZL] spielnicht:
I've gotten some PMs on how to downgrade from 4.2.2 (10.3.A.0.423) back to 4.1.2 (10.1.A.1.434 or 10.1.1.A.1.253), and thought I'd go ahead and post the steps for all to use. This works for both locked and unlocked bootloaders, and obviously keeps their state intact. The basic idea here is to use Flashtool to simply flash the stock FTF from Sony, no more, no less. It is the same process as restoring your phone via PCC/SUS/Mac Bridge, with the exception here that Sony isn't forcing the latest firmware on you and thus allowing you to select your choice of firmware version.

Rooting Notice

I know for certain you can root the 434 (4.1.2) firmware via Bin4ry's method, and I know for certain you can root the 423 (4.2.2) via DoomLoRD's method. However, I'm fairly sure you cannot root the 253 (4.1.2) firmware via any available method, unless you update from 434 to 253 via an OTA update...which is no longer available. So, if you need root, flash 434, if not, then 253.

The Standard Disclaimer:

I am not responsible for any damage that may happen to your phone by following these instructions. Do so at your own risk.

What You Need:

Flashtool (Official Site)

Correct FTF that matches your device's customization (e.g. Generic US 1270-7584)

To check your customization, dial *#8#7378423#*#*, Service Info > Software Info

You can search this forum for FTFs posted by fellow users, or get them from here. You can downgrade to 434 or 253, it's your choice. Just keep in mind that if you go back to 434, you won't be able to update to 253 via the OTA, you'll have to use this method again.

Before You Begin:

BACKUP ALL YOUR DATA!! When flashing the stock FTF, your phone is wiped 100% clean, as if right out of the factory. Even if you have a custom recovery installed, that will be gone as well.

Flashing Instructions:

Switch off your phone and make sure the USB cable is unplugged from the computer (leave it attached to the phone)

Make sure all Sony software (PCC, SUS, Mac Bridge) are closed and not running on the computer

With the phone completely off, press and hold the VOLUME DOWN button and plug the USB cable to the computer.

You're not going to see anything happen on the phone, but this will put it in Flashmode

Fire up Flashtool

It should say phone is connected in Flashmode

Click on the lighting bolt icon (Flash Device)

When prompted for Flashmode or Fastboot Mode, select Flashmode (default selection)

Navigate on your computer to where the FTF is stored

Flashtool will then display the details of the FTF

When flashing a stock FTF, make sure you keep Wipe Data/Cache/Appslog checked, and all Excludes unchecked.

Click Flash

At this point, the Flashtool will start the flashing process and should take anywhere between 10-15 minutes

If the Flashtool errors out or crashes, 9 out of 10 times you forgot the Sony software running in the background. Close, and try again.

After it's done, simply unplug the USB cable and start the phone.

The phone might prompt you for an NFC update, followed by a Power Off or Use Phone selection. I usually select Power Off and start the phone back up again to make sure the update completed all the way, but that's just my paranoia :D