Would it matter for the attack if you use this command card to move out of line-of-sight?

I assume the rulebook would give you the answer. My guess is that the target moving out of line of sight before step 7 of the attack makes the attack miss .

We used to speculate losing line of sight during an attack would abort the attack, but I don't remember if we had any actual cases in mind before.

Edit: is confirmed from the rulebook.

Miss allows surges to be spent on recover and any non-blast, non-cleave, non-condition keywords, and perform after the attack resolves abilities, unlike an aborted attack.

Yeah, I felt that the current (pre-Jabba) rules were lacking in this regard.

I guess that there must already be some rare combinations that allow a figure to move between an attack is declared and resolved, not that I can think of one right now, but they seem more of a corner case than what will be central to what the new command card On The Lam does.

Okay this could be a pre-On The Lam situation?

Greedo declares an attack on an enemy. The enemy attacks, and manages to hurt Greedo, and plays Opportunistic (also being Scum) moving out of line-of-sight.

Actually, I guess this could be the case with a number of other situations with Opportunistic. For instance if attacked by a Gamorrean Guard using Labored Attack, eating the damage instead of discarding a card.

And this was indeed cleared up with a new rule entry under Attack in Jabba's Realm, that if a figure is out of sight during the attack, then the attack misses.