Hey @James, long story short, there's issues with some S3's being un-downgradeable after the update to Android 4.3 (thanks Knox!). I don't recall off the top of my head if the T999 is one of them. I believe it was the AT&T version, not the T-Mobile one...
I would suggest checking out this XDA Developers thread about switching 4.3 ROMs to solve issues. You may be better off just switching Jelly Bean ROMs than downgrading and possibly facing bricking issues.
As for your friend's comment, he means that your flash counter has increased by one. This is a trick the manufacturers use to know if you've been flashing firmware on your own (usually so they can deny you your warranty and service). But you can reset the flash counter to zero (not on all devices though). Here's a tutorial on reseting the flash counter to zero. The problem with that is if you have Knox already you can't use Triangle Away to reset the counter - this means your warranty is gone, so you may as well flash a custom ROM running on the 4.4 code base instead!
Let us know if none of this makes sense!