Invert Polarity

If the target spell has a variable number of targets, you can't change how many targets it has.

If the target spell has damage divided as it was cast, the division can't be changed although the targets receiving that damage still can. The same is true of spells that distribute counters.

If you choose new targets for the target spell, the new targets must be legal.

You don't make any decisions about new targets for the spell until you win the flip.