Default [Q] Going to upgrade from an S4 to the M8, and I'd like to know

if HTC allows downgrading the bootloader so I can install a custom rom after I make the mistake of taking an OTA update from ATT. With the S4 this wasn't possible. but since this is my first android phone, I was wondering if it was Samsung trying to keep the device "secure" or ATT forcing these companies to keep their bootloaders locked.