Re-locking the bootloader is not needed with s-off
S-off will allow to to flash what you want ...but not without some other steps involved. Your phone must still match the firmware / RUU your trying to flash. You must change you MID and CID to match that of the RUU
Lets convert to .401 international
Download these 2 files
Firmware >>
http://xda7.androidrevolution.org/db_mirror/Firmware/HTC/HTC_One/401/4.19.401.9.zip
RUU>>
http://www.androidruu.com/getdownlo...10.38r.1157.04L_release_353069_signed_2-1.exe
and you'll need this MID_Change_1.01.zip
https://www.androidfilehost.com/?fid=23501681358538634
The MID tool works in custom recovery so do it first
change your MID to modelid: PN0710000
(If that's already your MID skip to the next step)
Next is your CID you change it from fastboot USB and the command prompt
fastboot oem writecid HTC__001
( or any of these will work )
Now you simple flash the firmware
fastboot oem rebootRUU
fastboot flash zip firmware.zip
fastboot flash zip firmware.zip
fastboot reboot-bootloader
and then run the RUU on your PC
you'll be on Android 4.3 Sense 5.5. and updates for 5.x and 6.x should begin