For some reason, you might want to downgrade your HTC One from Android 4.4.2 to Android 4.3. For example, I updated my AT&T HTC One to Android 4.4.2 with Sense 6.0 by converting it to Developer Edition. After using it for some days, I wanted to revert it back to stock conditions. You might want to do it before returning it to AT&T for warranty or other issues. If you just try to flash a lower version RUU, you will most likely get error message. The problem results from the fact that higher Android firmware from HTC usually comes with a higher version of HBOOT. So, whenever you try to flash an RU.exe, it will show you error as it cannot flash a lower version HBOOT to a device which already has a higher HBOOT version. We need to manually flash a lower version HBOOT first and then use RUU.exe to return to complete stock.