I'd love for it to work that way, but you declare who the boost is against when you declare boost, before you measure for ZoC. By the time you are at the resolution step where you measure to see if they were actually inside your ZoC, it is too late to declare another ARO target, is it not? To be fair, this is not the first time this has come up, and playing it the more logical way (choosing a target that is in ZoC if your first choice happened to be outside), seems to be the consensus on how it should be done. The problem is that the rules are written contradictorily in this case. In the most general sense, how is the obligatory tag supposed to be played when this line is in the rules: "If a trooper can declare an ARO but fails to do so, the chance is lost"?