From P&P:
If you have to declare this when the Shadow ship moves, the fighters will just run away. It still has its uses, against ships which can dodge or after moving double speed to get next to an enemy ship which you can't now attack with the beam in normal mode because you're facing the wrong way. But it does make this "anti-fighter defence" useless against fighters.
But if you are allowed to declare that you're converting your beam into anti-fighter mode right at the end of the Movement phase, after fighters have moved, then it becomes much more useful against fighters. Are you in fact allowed to do this?
It doesn't say exactly when during the Movement phase, though.During the Movement phase, you may opt to convert the Beam weapon into one that will devastate enemy fighter formations.
If you have to declare this when the Shadow ship moves, the fighters will just run away. It still has its uses, against ships which can dodge or after moving double speed to get next to an enemy ship which you can't now attack with the beam in normal mode because you're facing the wrong way. But it does make this "anti-fighter defence" useless against fighters.
But if you are allowed to declare that you're converting your beam into anti-fighter mode right at the end of the Movement phase, after fighters have moved, then it becomes much more useful against fighters. Are you in fact allowed to do this?