Spell Master, Variant (5e Feat)
From D&D Wiki
Prerequisites: The ability to cast at least one spell
When you cast a spell with a spell slot and the spell deals acid, cold, fire, force, lightning, necrotic, radiant, or thunder damage, you can substitute that damage type with one other type from that list (you can change only one damage type per casting of a spell). You may use this feature four times, and you regain all expended uses when you finish a long rest.