Once you hit 4.3, you lost your ability to downgrade. You can upgrade, but you can't go back. Rest easy, as long as you can get into download mode, you can recover your device. Bootloop, soft brick, whatever; as long as Power + volume down brings you to download mode everything is still OK. Just read the directions a few times.. get used to the idea of what's going on, double check and make sure you have the right software and files, and jump in. You'll do fine. Even after years of jailbreaking iphones and rooting Droids, I still get nervous whenever I have to fire up Odin; but just remember... many many others (more than have posted in that thread for sure) have done the same thing you're about to do, so as long as you follow the directions you will be fine!
Edit: Can i ask you a favor? Could you get me the "M919UVUFNB4_M919TMBFNB4_TMB.zip" modem file and "M919UVUAMDL_M919TMBAMDL_TMB.zip", i think that's what the tutorial link i posted is asking for. But the files r huge, 1.6gb, etc, would take me 3hrs+ each for each file to download just to extract the modem =/ If that's the only thing i need.
found these links: http://rapidgator.net/file/59487dc51...15123.zip.html
and : http://terafile.co/32d149264215/M919...MBAMDL_TMB.zip
Not sure if they are safe to use or not, not everything on the net is...plus they r huge files =/