In order to figure out the answer, just keep reading a bit past where the word "overhaul" actually occurs. Basically what is going on here is that the soldiers are changing their bayonets to take off the saws (serrated edges) that some of them have on their blunt sides. They are doing this because they believe that the enemy will kill them (and probably in a particularly nasty way) if they catch them with the bayonets that have saws on them.
The idea here is that the bayonets with saws are seen as a nasty weapon. Because of this, the enemy has been killing people caught with those bayonets instead of taking them captive. Soldiers don't want this to happen to them and so they "overhaul" their bayonets.