In general, most updates going from one version of Android to another will break root. If you are concerned with that just wait for a rooted version to be released and flash that. Otherwise, wait to confirm that root can be achieved with the latest 4.3 update so you can re-root if necessary.
To add to this, if AT&T has pushed the update to your phone, your phone is nagging you to update and may have (hopefully not) updated on its own. I have found a way to insure it won't download the update nor try to update ->
(If your phone has already downloaded the update and is nagging you to preform the update, follow step 1 first)
1) With your favorite root browser, go to /cache/fota/ and delete anything that is in this folder.
Now, regardless if your phone has downloaded the update our not:
2) With your favorite root browser, go to /cache. Select fota and set the permissions so that the owner has read permissions ONLY. No other permissions should be checked for the owner or anyone else.
This should keep your phone from being able to download the update. Once there is a known way to root 4.3 on an AT&T s4, go back into your cache folder and change the permissions for the fota folder back to read and write for the owner, then preform the update manually through settings-> more-> about device-> software update-> check for update.
I did this little trick to my phone the day the update for pushed to my phone and have had no issues.