2014-11-24 : If an effect creates a copy of Pyrotechnics, the number of targets and division of damage can’t be changed. The effect that creates the copy may allow you to change the targets, however.

2014-11-24 : If some but not all of Pyrotechnics’s targets become illegal, you can’t change the division of damage. Damage that would have been dealt to illegal targets simply isn’t dealt.

2014-11-24 : You choose how the damage is divided as you cast the spell, not as it resolves. Each target must be assigned at least 1 damage.