When did the CSI Las Vegas team shift from forensics only to carrying weapons, interrogating witnesses, etc.? Was the procedural change duscussed on show or just a shift in a particular season and not explained?

It wasn't directly addressed on the show, but I'd assume the change took place around the time CSI: Miami aired and gained popularity. Miami was known for the big action sequences and gun fights. I think this CSI wanted to go the direction they were going, and it also gives the characters much more to do.