The first thing the update program does is check that the ROM is the one it's supposed to update. Then it checks a few things to see if the ROM has ever been rooted. If it has, the update program stops. Updating a ROM that's been rooted, even if it's not rooted now, can brick the phone.
What you have to do is flash a stock ROM, then update it. Check to make sure that there's a rooting method for that phone for the new version of Android you're updating to. (Each version has the older methods patched out.) If there's no way to root the current version yet, it's your choice whether to stay on an older version (I'm still on 4.4.2) or update and not have root.
(I doubt it has to be rooted to work on your carrier - you just have to have it SIM-unlocked. If that's been done, it stays unlocked even if you update the ROM and you're not rooted.)
06-05-2015 04:32 PM