Looking at this spell using SpellWork, (Spell 78: Heroic Strike) Changing it from Next Melee Attack to Instant cast would break the function of the spell. It adds 11 to the next swing of the weapon, shifting it to an instant cast would mean it only does 11 damage, ever. you would have to add a second effect to actually deal the normal weapon damage.
But to change it from an instant cast, change the Attributes column from 0x50014 to 0x50010, this will make it a stand alone spell.