A big discussion is occurring on the reddit board as to when targets have to be chosen. In particular, do you have to indicate the target of a card during the pre-initiation phase or only when the card commences being played (step 3 in the Initiation Sequence: Appendix I of the Rules.)
Example: Say you are engaged with an enemy and have Brother Xavier with 0 damage and your partner has 2 damage. You play Emergency Aid. Do you have to specify that the target is your partner (the only legal target at the time), or is the existence of a legal target enough... because if the existence of a legal target is enough then what could transpire is that you take an Attack of Opportunity, assigning the damage to Xavier, and then when the card actually commences you could now target Xavier, who has damage on him.
I think the rules strongly suggest you have to declare the target in the pre-initiation phase. And for two reasons:
- During the pre-initiation phase the rules say you must "declare your intent," and I would say that selecting a target counts as part of the declaration of intent.
- To confirm that the card can change the game state, you have to know beforehand what the targets are. If simply having one target that would change the game state is sufficient, then there is nothing stopping you from eventually selecting another later that would not cause a change of state.
However, I could see another case being made that says "there is a check that the game state could **potentially** be changed during the pre-initiation segment, but then when a target is actually selected, there is another check that that target actually does change the game state (see rules on Targeting), so having someone wait until the card resolves does not compromise the premise that cards should change the game state.