Whenever you cast an instant or sorcery spell, you may copy that spell. You may choose new targets for the copy.
7/14/2017 Swarm Intelligence can copy any instant or sorcery spell, not just one with targets.
7/14/2017 Swarm Intelligence can copy the spell even if it’s countered before Swarm Intelligence’s triggered ability resolves.
7/14/2017 The copy is created on the stack, so it’s not “cast.” Abilities that trigger when a player casts a spell (such as Swarm Intelligence’s own ability) won’t trigger.
7/14/2017 The copy will have the same targets as the spell it’s copying unless you choose new ones. You may change any number of the targets, including all of them or none of them. If, for one of the targets, you can’t choose a new legal target, then it remains unchanged (even if the current target is illegal).
7/14/2017 If the spell that’s copied is modal (that is, it says “Choose one —” or the like), the copy will have the same mode. A different mode can’t be chosen.
7/14/2017 If the spell that’s copied has an X whose value was determined as it was cast (like Torment of Hailfire does), the copy will have the same value of X.
7/14/2017 If the spell has damage divided as it was cast (like Chandra’s Pyrohelix), the division can’t be changed (although the targets receiving that damage still can).
7/14/2017 The controller of a copy can’t choose to pay any alternative or additional costs for the copy. However, effects based on any alternative or additional costs that were paid for the original spell are copied as though those same costs were paid for the copy.