Augmentation

When a magical effect that requires a target to make a saving throw such as, stunning strike, breath weapon or a spell.  the caster can use their reaction to manipulate the magical effect to change the type of saving throw required(making stunning strike a WIS saving throw instead of a CON save). this spell must be used as the magical effect is used and the change in saving throw must be plausable at DM's descretion(making firebal a STR save makes no sense). 

it is up to the caster to describe how they are changing the spell to incur a different saving throw.