2013-09-15 : If an effect creates a copy of Boulderfall, the division of damage and number of targets can't be changed. The effect that creates the copy may allow you to change targets, however.

2013-09-15 : If some but not all of Boulderfall'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.

2013-09-15 : You divide the damage as you cast the spell, not as it resolves. Each target must be assigned at least 1 damage.