[AT&T, Rogers, Bell, Telus Samsung Galaxy S III] JOrion:
Hi .. I got the stock firmware from SamMobile www(dot)sammobile(dot)com /firmwares/database/SGH-I747/ATT (sorry cant post outside links yet)

this brought me back to stock 4.1.1 ... and changes all the information in the phone to the 747 and also it will be recognized as d2att instead of d2can so roms seemed more compatible. You will not be rooted but if it will still be "carrier unlocked". It wont download updates from Samsung because the flash counter has already been tripped. I believe installing this rom will reset the flash counter but it still keeps it at 1 if I remember correctly.

Not sure if you have oden set up or anything yet .. but if not .. just do a search on rooting your phone in these forums ... there are plenty of step by step posts... just follow one to the point of getting oden installed and connecting to your phone ... then put your phone into download mode .. open oden ... click the pda button and load file you download from sammobile... make sure that reboot and F.Reset Time checked and making sure that Re-Partition is unchecked. Start it up and wait.... you phone will eventually start to boot again but don't unplug it or do anything yet... wait till Oden says Passed and the little bar at top turns green. ... I just left it plugged in until it finally booted into the OS. ... it does take a while and will look like its hung with the white screen and AT&T icon... just wait .. let it go .. it will get there eventually.

If you want to root again ....

The process I followed was to download TWRP (openrecovery-twrp-2.8.1.0-d2att.tar) www (dot) teamw (dot) in /project/twrp2/104

Then I downloaded TeamEpic-Root-from-Recovery-v5 www (dot) teamw (dot) in /project/twrp2/104 and copied it to my phones sd card.

I flashed TWRP using Oden leaving reboot and F.Reset Time checked and making sure that Re-Partition was unchecked. This will void warranties but as for us I guess we didn't have any anyways .. since the flash counter has already been tripped. I believe it tries to reboot after it finishes flashing TWRP .. if so pull the battery .. don't let it boot back into the OS.

I put the battery back in and booted into recovery. Using TWRP, I flashed TeamEpic-Root-from-Recovery-v5... Did a factory reset in TWRP (Wipe button in TWRP) .. rebooted and I was at 4.1.1 stock rooted.

There are places on this forum where you can get 4.3 and even 4.4.2 installed .. Im running stock KitKat right now but I had issues with it that are big and now that I had them I found that a lot of people have problems with the sdcard not being able to be written too in KitKat .. I tried to downgrade to 4.3 and soft bricked my phone ... well not really I just didnt realize that its very hard to downgrade your bootloader and modem ... the problem I had when I downgraded is .. it left my bootloader set for KitKat and changed my modem to JB. I lost my carrier, my baseband and my IMEI I had to find a modem to flash that was I474UCUFNE4. Even then I had problems .. I'm back on KitKat right now "without the no writes to sdcard bug atm" but I still haven't installed everything.

I'm basically new to flashing too .. did it a while back for the MyTouch 3G Slide ... If I was you .. after getting the stock firmware rooted .. that would be as far as I would go ... Im kicking myself for going with 4.3 and 4.4.2 because now I'm stuck ... I can't get 4.3 to flash correctly and work .. I can't find any way to down grade my boot loader and modem and everything so it is back to the 4.3 stuff .. It doesn't look like I can flash any of my backups I made for device. I haven't tried it but I doubt I will be able to use any custom roms because they usually look for the bootloader and usually only looks up to the 4.1.1 - I747UCDLK3 so when it sees my I474UCUFNE4 it fails. So stick with 4.1.1 .. if you want kitkat try cyanogenmod CM-11 .. once you get your phone back to stock 4.1.1 it will be able to be flashed .. it doesn't update your bootloader or modem it will run off the I747UCDLK3 bootloader and is available for the d2att. As mentioned by someone else .. if you go to the 4.3 or 4.4.2 roms ..you have to update your bootloader and once that is done .. it will not be able to go back and if you try to say reinstall this stock rom to get it back you will have a brick because you cant downgrade 4.3 to any lower bootloader .. 4.3 does something to the phone that makes the older loaders even if reinstalled, completely unusable and will cause a hard brick you will probably not be able to fix yourself.

Good luck hope this helps