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.

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