The AT&T version of the LG G Flex is getting upgraded! Huzzah! But don't bust out the champagne just yet, G Flex owners: at the end of your upgrade you'll still be using Android 4.4.2, like you have been since April. According to AT&T's update support page, the newest patch for the phone doesn't add very much at all.
G Flex software build D95020f adds the "latest Android security patches," an updated version of LG's built-in music app that you don't use, and some compatibility fixes for the LG G Watch. (AT&T sells the G Watch in its retail stores - I suppose either they or LG have been getting some complaints.) AT&T is also tweaking its APN file. Aaaaaaand that's about it. At this point, I'm guessing LG and most of the other manufacturers are waiting until Android L to send out the next big update.
AT&T doesn't say how big the OTA file is, but the official instructions include the directive to connect to Wi-Fi. As always, if you've done any root-level modifications to your G Flex's software it's possible that the update could cause errors, or more likely, just fail to flash. The update will probably go out in batches, so don't be surprised if you don't see the alert for a week or so.
Source: AT&T support