So, let's say we want a mage character based on ice, sure, there are quite a few spells with Cold damage/thematic, but they're not quite what you want... You want him to have a Fireball that deals Cold damage, you want a Frostball, Iceblast, Frost Explosion, call it whatever you want!
So, if I took spells like Fireball, and changed the damage type to ice and the saving throw to CON, would that affect the game balance?
Another example, let's take Sunbeam and turn it into... Darkbeam? It now deals necrotic damage instead of radiant.
I'm aware of this question: What are the most and least-resisted damage types?, I know that a Fireball that deals Psychic Damage and requires an Int save is stronger.
So let's classify saves in two categories: The "common", Dex, Con and Wis, and the "exotic", STR, INT, CHA.
So, if I swap "similar" damage types (fire/cold, necrotic/radiant, etc) and also swap saving throws of the same "category" (Dex => Con/Con => Wis, etc) would that mess up game balance?