Can I change my bootloader on my S4?

Hello all. I just got a new Galaxy S4 (i337m) and I hate the stock firmware on it. I want to install something custom, however the custom roms require a certain bootloader that I do not have. I have searched for if / how I can change my bootloader but none of the forum posts I have found seem to be related to my specific phone so I'm posting this to find out about my specific phone. I believe it is a locked Fido phone. I used the ADB shell to check the bootloader version and it is not the UCUAMDB or UCUAMDL bootloader I need to have for the roms Im interested in. Mine was something like CUVL. I can verify later when I get home. However, can I change my bootloader? If so, are there any instructions for this specific phone? My goal for this phone is a light weight / vanilla version of 5.1.1 Lollipop, rooted. Thanks.