The ability can target any instant or sorcery spell, even if it has no targets.

If the spell has multiple targets, you may either change all the targets or none of them. Each target is treated individually, and must be changed to a different legal target. For example, Seeds of Strength targeting Atog (target #1), the same Atog (target #2), and Scryb Sprites (target #3) can be changed so it targets the same Scryb Sprites (target #1), Eager Cadet (target #2), and Alpha Myr (target #3). If changing one of the targets would be impossible, then you can't change any of the others.

If a spell has a variable number of targets (such as Electrolyze), the number of targets chosen can't be changed.