Help Haven't changed my bootloader


Hey guys, so Ive had a S4 (M919N MetroPCS variant) since it first came out. It originally had Jellybean (4.2.2) installed when I bought it. Long story short, I upgraded to AOSP KitLat 4.4.4 and then later upgraded to AOSP 6.0.1
But, I checked out my baseband on my phone and it says "M919NUVUAMF2". This indicates that I still have the Jellybean modem correct? And what are the consequences if I don't get the suited modem/bootloader for my device when upgrading unofficially. it hasn't affected my phone in any way and I've been going back and forth on ROMS from AOSP to TouchWiz varying from KitKat to Lollipop to Marshmallow. Is it essential for me to change my bootloader when flashing a new rom?