It seems clear to me that some folks come from a Magic background, so timing issues have been drummed into them as being of paramount importance. However, X-Wing does not have the same level of hard requirements for timing, you are allowed to change your mind after checking range/arc/etc.
There is no reason, other than habit of mind, to separate target selection and weapon selection. The active player decides who he is shooting at and with what. If it helps the MtG players, consider it playing a card and choosing a target. There is no intermediate step in between the two, both are performed at the same time and cannot be separated. Whether or not the target "becomes" illegal after that is immaterial, a legal target is chosen when the card is played or else the action simply does not occur. Even in MtG, you cannot pay the cost of an action if the action itself is illegal, it simply does not happen.
So, the attacker chooses his target and the weapon he is using to shoot it with simultaneously. If the weapon system cannot hit that target for some reason (out of arc, cannot spend a focus token, out of range) then he picks a different target to shoot at with whatever weapon system he chooses to shoot it with. He is not "locked in" to the weapon system he wanted to use in the first instance, neither is he "locked in" to that target but forced to use a different weapon. His first target/weapon combination was illegal (or he just decided to do something different), so the action is voided and he just starts over from the beginning.
Edited by KineticOperator