The AI you need to kill to continue will try to shoot you if you get too close to them. I think there's 4-6 of them.
Some AI (such as the marines) will try to shoot you if you kill four of them in a short chain and get close to another one.
There's no graphical changes but if you really need a picture, here's one.

Since the Marines aren't chasing after the Covenant, this is what they do in their spare time.
By the way, HexEdit doesn't change "Date Modified" for some reason so it'll still appear to be "September 23rd, 2003" instead of some date in the year 2006.
http://www.halodemomods.com/wiki/File:AI_Ignore_All.zip