[AT&T HTC One (M8)] pavankvn:
I converted my AT&T One M8 to US unlocked version (CID: BS_US001) using the directions in the thread below:

http://xdaforums.com/htc-one-m8/general/change-wwe-m8-to-dev-edition-ota-t2826567

This assumes you have S-OFF, and unlocked bootloader and i started with an AT&T stock rom with no root. I am not sure if that is a requirement. For this i did the AT&T RUU by installing an older stock recovery. (AT&T 4.4.3 RUU)

1. Change CID to BS_US001 - open command prompt on your computer and navigate to your fastboot folder; alternatively open the fastboot folder and hold down shift and rightclick, and select open command prompt here. turn USB debugging ON on your phone and connect it to your computer in fastboot mode (turn your phone off and while holding the VOL down button press the power button, the phone goes to bootloader mode, select the fastboot [first option] and press power button, connect the phone to computer and it should say fastboot-usb), verify connection by typing fastboot devices in cmd prompt, it should give your device code. after confirming you are connected via fastboot, type "fastboot oem writecid BS_US001" Reboot bootloader and confirm CID change by typing "fastboot getvar all" and your CID should read "BS_US001".

2. Restore developer nand - for this you will need a custom recovery like TWRP or CWM. If you already have TWRP, then please proceed. To install custom recovery, download TWRP from here. save the twrp recovery image file to your fastboot folder. put your phone in fastboot mode and connect to computer and open cmd prompt from your fastboot folder and type "fastboot flash recovery recoveryname.img", recoveryname should be exactly what the same as TWRP recovery filename. Verify TWRP installation by typing "fastboot reboot-bootloader" and selecting recovery. TWRP recovery should load. Download BS-US001 stock nand from here. At this point boot your phone normally and connect to computer. The navigate to the TWRP folder and backup folder, and paste the BS_US001 nand zip file here. Reboot to TWRP recovery and select restore and select the BS_US001 file you just copied.

3. Install Unlocked version (BS_US001) firmware which includes stock recovery - Download the unlocked version firmware from here. For installing firmware i used this thread. There are two methods however, i used the following method and is my favorite. For this it is recommended that the HT sync manager be uninstalled leaving just the HTC drivers. Place the firmware zip in the fastboot folder. Put your phone in fastboot mode and connect to computer. Open cmd prompt in your fastboot folder. Type "fastboot oem rebootRUU", phone will turnoff and a dark screen with only HTC will appear (this is the RUU mode), now type "fastboot flash zip firmware.zip" replace firmware with the actual filename of the firmware zip. You will see a green progress bar while the firmware is being flashed. I read somewhere that the flash will fail the first time so reflash the firmware a second time by typing "fastboot flash zip firmware.zip". It did fail for me the first time and installed the firmware a second time and it worked. type "fastboot reboot-bootloader" from here select option to reboot your phone normally. Once booted and connected to the internet, you will receive an OTA update notification for 4.4.3. After installing that, another OTA for GPS improvements will appear.

This worked for me. If you are stuck at some point, please read through the original threads. Especially the firware installation thread has a nice FAQ. Goodluck. Please remember, you can not install an OTA while you have a custom recovery. Please wait until you install the firmware zip for accepting the OTA. If the firmware does not install fro some reason, you can pull the stock recovery from the firmware zip and install this recovery using fastboot. Then you can accept the OTAs.

Let me know how it goes.:o