I had that happen to my Note 3 - an OTA update wiped it out (because the battery died during the update and their software makes Hoover look bad). I complained. I screamed (not literally.) Because there were no refurbs yet, they replaced the phone with a new one.
You break my phone, you own my phone, whether you're the kid down the block or Cingular (who actually owns the AT&T Mobility trademark).
With the S3 with the 4.3 update you might not find it so easy to flash a ROM, not even the stock ROM. It depends on the carrier. (AT&T and Verizon have locked bootloaders. And they all have Fort Knox.)