I've thought of the idea of having spellcasters that can replace the element type of a spell with another elemental subtype, e.g. Allowing for fireball to become a frostball, poison ball etc.

I've searched around quite a bit to see if there is anything like it from before, but no luck so far. What is peoples general thoughts about something like this overall? For instance, limiting the replaced energy with fire/cold/lightning/thunder/poison, aka the ones that have the highest amount of resilient enemies in the first place, and avoiding the stronger elements like Force, necrotic, radiant and psychic.

I don't have a lot of experience with this... Although I did have an 'Icy' Tiefling, whose fire attributes/spells all changed to cold after making a pact with a goddess that deals with ice. Even though more enemies have fire resistance, the change didn't seem to make a major difference in their fighting abilities.

I could see there being issues with meta gaming, changing a spell to a weakness even though the character couldn't know the enemies weakness. I think maybe making it a limited ability, something you can do only a few times a day, would probably stop it from being OP if that were to be a problem.