Normally when learning a spell, the spell is set in regards to its damage type. Given the way making new spells work, if a player decides that they’d rather have a fireball that deals acid damage, I do not see an issue with making this the default at this time. The rarer energy types like radiant, necrotic, and psychic are not allowed. The thunder type should be considered.
If a player wishes to make this change to a spell, it is a semi-permanent change as it is not on the fly but when the spell is learned. A wizard would have to have both fireball and acidball in their book to do such a thing. Each energy is its own unique spell in this rule.