Literally all you need to do if you are downgrading is use Revone to obtain S-Off and even if you aren't downgrading you can still use it to stray away from Relocked and Tampered bootloader. Use Revone to relock the bootloader in a way that it shows "Locked"(-t -l) and not relocked or tampered. Run RUU and immediately after completing RUU session just use "adb reboot bootloader", "fastboot oem writesecureflag 3", "fastboot reboot"
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Sprint HTC One(Unrooted)
Upgraded From Sprint Galaxy SIII, HTC EVO 4G LTE and Google Nexus 4 16GB(Unlocked)