Theming Spellcraft; A player's guide.

Or, this is intended for use by players. The first ~page or so is a justification of why this works as well as it does, and an explanation of how to handle it for DMs. The next few will go into depth on the exact mechanics I've thought of for players.

Why I have no qualm changing damage types freely. Looking at a bunch of spells, particularly first level ones like Chromatic Orb, Guiding Bolt and Catapult, before effects riding on the spell alongside the damage become so varied in their own power levels that it's impossible to tell... All these spells deal the same damage. Chomatic Orb, which gets to choose and literally will never be resisted because Thunder is one of those damage types. Guiding Bolt, which doesn't have the versatility to hit weaknesses, but does have a slight but noticable boost on every casting. Catapult, which has neither of these advantages and merely does Magical Bludgeoning damage. However, when one changes the delivery method or riding effect, the spell's damage changes. Dissonant Whispers, for the first example, has a much more powerful effect riding on the spell than anything the previous three brought to the table. I'd say this is why it deals 1d6 less than Guiding Bolt. Frostbite targets Constitution, a score monters tend to have in spades. Compared to Vicious Mockery, targeting the relatively rarer Wisdom, reveals the reason these two spells have a different damage dice. And, even then, it's the damage dice size, truly a slight difference. Update: Mike Mearls confirms this.

The one exception to this; As I mentioned before, it seems the type of saving throw a spell uses in part determines it's damage dice. It's... surprising... that this is more effective than damage type itself in WotC's eyes, but hey, that fact is also convenient. So... It's an exception to my general stance here, but I, personally, wouldn't allow a player to copy a spell and change it's type of saving throw. (Or, from an attack roll to an XX saving throw) It should be easy enough in most cases to explain why one type of save or another works with your new chosen element.

How should a DM handle this? I am making this supplement for flavour-focused characters to shine. No longer shall Acid mages languish with no spells to select on level up. No longer shall frost mages look over at their Fireballing bretheren in envy. And, I am a person very biased against min-maxers. They don't truly damage a game, and are easy enough to compensate for, but I hate them. And, it's not what I made this for. As such, the first question I would ask a player who wants to use this, is "Why do you want to use this?" If they respond with something along the lines of changing all their damage to Force so that nothing resists them, it's well within your powers to nope that. However, if they explain how it makes sense for their Jedi Knight to occasionally use Force spells alongside supportive ones, this is a completely different story. (Just an example using the same damage types, you by no means need to allow Jedi in your setting :p.)

But interaction with class features? And? Most subclasses like Dragonic Sorcerers or Evoker wizards have some kind of alternative they COULD be using which would work just as well. In the vast majority of cases, allowing interactions with class features is perfectly fine. The only possible outlier is the tempest cleric, who has a... limited... selection of lightning spells over their tenure, a major thing keeping their channel divinity in line. However, for their case, I would reccomend, storms are not only comprised of thunder and lightning. Get some hail up in there, some spikes of swirling wind, some sleet, Bludgeoning and Cold are also very thematic damage types to use.