Flashing a Bootloader

[j0hn13y]

No, i am not talking about custom recoveries here. i'm curious as to what would be the steps in replacing the bootloader on most phones. my general assumption of a bootloader is that its kind of like a BIOS, it warms up all the bits then hands over control to the OS. since BIOS flashing is usually pretty simple, would this also be simple, or would it require swapping chips?