Use this to downgrade 4.3 to 4.1.1 - flash using custom recovery
Then use these instruction to unlock your phone.
Put a different SIM to your own into the handset and then turn off the handset and switch it back on with that SIM inside.
Go to the dial pad on your phone and then type in *#197328640#.
Head over to the Main Menu of the device >  UMTS >  Debug Screen >  Phone Control >  Network Lock > Options Perso SHA256 OFF > and wait for around 30 seconds.
You should then head to Menu and choose Back, where you will come across the Network Lock. Choose  NW Lock NV Data INITIALLIZ.
That's it your phone is now unlocked. You can now use any sim card.
You can update, Root, flash custom rom and everything you want. It will stay unlocked