Default [Q] How to Network Unlock Galaxy S3 sgh-T999

I'm a bit of a noob, so apologies if this is in the wrong section. I've had a very difficult time finding threads that focus on those who have (stupidly!) applied the OTA 4.3 jellybean update, and what they can do to network unlock the phone. So here goes:

Samsung Galaxy SIII
SGH-T999
Android Version 4.3
Baseband Version T999UVUEMJC

Stock build. No root.

The closest step-by-step I've come across:

Remove T-Mobile SIM. Replace with ATT SIM. Reboot phone.

*#197328640# which successfully puts me into service mode.

This brings up the following menu:

MAIN MENU
[1]UMTS
[2]CDMA
[3]LTE
[4]SIM

From there its
[1]UMTS
[1] DEBUG SCREEN
[8]PHONE CONTROL
[6]NETWORK UNLOCK
[3]PERSO SHA256 OFF ...

Now, that's as far as I can get. The guide then says to go back to [6] and there should be an option, [4] NV INITIALIZZ.

This option is not there for me. (Note: at this point the PERSO SHA256 status is indeed set to SHA256_ENABLE_FLAG [0]).

Another guide suggested BACKing to the MAIN MENU and selecting option [5] run EFS SYNC() after selecting [3]PERSO SHA256 OFF.

I have done this and the screen reads: SYNC success. please reboot.

I do so, heart aflutter... only to be met with a SIM unlock PIN code entry screen.

Unlock fail.

Other threads have suggested downgrading to 4.1.1 in order for this to work and the necessary options be made available. Most of what I have read has said that attempts to downgrade from OTA 4.3 (with big brother KNOX) will result in a hard brick.

So, if you've made it this far... Has anyone figured out how to SIM unlock this particular phone running OTA 4.3 without rooting/downgrading?

If there is a more suitable thread discussing SIII's that are running 4.3 please direct me there.

Many thanks!