Did the method work? I converted my AT&T HTC One to developer edition and upgraded it to Android 4.4.2 with Sense 6.0 with bootloader locked and S-OFF. Consequently, mu HBOOT was 1.57 and when I tried to flash the 4.3 RUU, it didn't work and showed error 155 or something like that. What I had to do for solving this was to flash HBOOT 1.55 (same as 4.3 RUU) and then run the 4.3 RUU. It succeeded after that.