Elemental Transmutation

Using your knowledge of the elements and the arcane you can alter a spell you are about to cast to deal a different damage type than the spell would normally deal. 

As a bonus action choose an option from the following damage types: acid, cold, fire, lightning, poison, or thunder

The next spell you cast before the start of your next turn that deals acid, cold, fire, lightning, poison, or thunder damage, deals the chosen damage type instead.