The last version of Lollipop, Android 5.1.1 finally approved by AT&T for the Galaxy Samsung Galaxy S6 and S6 Edge, both devices are getting an update at the same time and getting the similar software. The OTA software version of the device is also quite similar, the original S6 comes with G920AUCU3BOI2, while the edge variant contains G925AUCU3BOI2 version. Unfortunately, not every user is happy with this update, as this Android 5.1.1 for the AT&T Galaxy S6 and S6 Edge could make the root attempt more challenging.
Based on some news from xda community, after successfully upgrading an AT&T Galaxy S6 and S6 Edge to Android 5.1.1 (OI2), you will lose root access. Given that the bootloader also been upgraded to the latest version, consequently gain back root is not possible at the moment.
Additionally, this OTA update will also prevent you to downgrade the firmware to the older version. So, in other words, we will be stuck with unrooted device until someone discovered a new method to gain root or even unlock the bootloader.
The Android 5.1.1 brings quite a lot of improvements and bug fixes, if you have experienced the memory leak issue and felt very annoyed with it, this update will be quite helpful to you. So, if you prefer a stable and smooth system, you can immediately update the phone the phone to OI2 via OTA.
However, if opt to keep root access, then you have to wait and keep living with the previous software. While rooted you still have an option to flash custom (stock) ROM, there are several custom ROM available on xda, you can try if you are bored with the Samsung’s TouchWiz.
There’s also a thread on xda who talked about the root possibilities for this Android 5.1.1 update (OI2) by trying some of the older root utility like Pingpong root, Kingroot, Kingo, etc. You can follow it and perhaps you can get the latest root updates there.