WoTC have released an Unearthed Arcana Wizard subclass called Lore Master. One of the features this subclass gives is:

Spell Secrets

At 2nd level, you master the first in a series of arcane secrets uncovered by your extensive studies.

When you cast a spell with a spell slot and the spell deals acid, cold, fire, force, lightning, necrotic, radiant, or thunder damage, you can substitute that damage type with one other type from that list (you can change only one damage type per casting of a spell). You replace one energy type for another by altering the spell’s formula as you cast it.

When you cast a spell with a spell slot and the spell requires a saving throw, you can change the saving throw from one ability score to another of your choice. Once you change a saving throw in this way, you can’t do so again until you finish a short or long rest.

This seems to cover both effects that you wish your Mage type character to have.

So WoTC appear to be looking at the same sort of concept that you want for your homebrew campaign.

Looking at this trait however it is evident that WoTC, at least, consider the changing of damage type to be not that big of a deal to game balance, but the changing of the type of saving throw to have a potentially greater effect (and thus needed to be restricted).

Taking your fireball example, changing the saving throw from DEX to anything else would eliminate the rogue’s evasion class feature.

In general the saving throws have been sculpted to the type of effect the spell is imposing.

Some salient examples are:

• area of effect spells are largely DEX saves (fireball)
• a spell effecting a targets mental faculties are usually WIS or INT saves (Fear or Feeblemind)

Important Caveat

It obviously comes with the caveat that this subclass hasn’t been playtested as extensively as the subclasses published in the books, and as a result is not necessarily balanced within the game.

You'll be fine

As the DM, you retain full control over the whole game world, including what spells are available and what they do. All you have to do is be honest and upfront with what rules you're changing before hand (and, if possible, why).

That's as easy as saying "Hey everyone. Before we get started, I'd just like to mention that Alex is interested in playing a darkness mage, so I've renamed Fireball to Voidball for Emomage the Wizard. It now engulfs an area with tongues of searing, dark flame. Everyone ok with that?"

This makes it clear that you're slightly tweaking the game and why. This should help clear up any confusion like "Why can't I find this spell in the book?".

The DMG has a few tidbits on changing spells. In particular, there's a section on page 283 called Creating a Spell. It offers some guidelines on damage, consistency with class themes (eg. no healing for Wizards), and tweaking the dice used. The closest it comes to a flavor change you're talking about is in the opening, where it says

When creating a new spell, use existing spells as guidelines.

Guidelines don't get much tighter than "mechanically indistinguishable".

There is some evidence of explicitly reflavoring a spell in the DMG as well. Take a look at page 41 in the Wuxia section.

... spells need only minor flavor changes so that they better reflect such a setting. For example, when the characters use spells or special abilities that teleport them short distances, they actually make high-flying acrobatic leaps. ... Flavorful descriptions of actions in the game don't change the nuts and bolts of the rules, but they make all the difference in the feel of a campaign.

The DMG is very clearly in support of the DM's ability to change this kind of detail to better support the campaign. The kind of change mentioned in the question, flames to dark-flames, is well in line with this kind of change.

The optional source book Tasha's Cauldron of Everything also features a section called Personalizing Spells on page 116, which explicitly supports cosmetic changes such as changing the color of a spell.

Just as every performer lends their art a personal flair and every warrior asserts their fighting styles through the lens of their own training, so too can a spellcaster use magic to express their individuality. Regardless of what type of spellcaster you’re playing, you can customize the cosmetic effects of your character’s spells. Perhaps you wish the effects of your caster’s spells to appear in their favorite color, to suggest the training they received from a celestial mentor, or to exhibit their connection to a season of the year. The possibilities for how you might cosmetically customize your character’s spells are endless. However, such alterations can’t change the effects of a spell. They also can’t make one spell seem like another—you can’t, for example, make a magic missile look like a fireball.

And it continues on page 117:

For example, the fireball of a wizard with a fondness for storms might erupt to look like burning clouds or a burst of red lighting (without affecting the spell's damage type), while the same wizard's haste spell might limn the target in faint thunderheads.

The sidebar notes that Tasha adds spectral chicken legs to her spells (in an effort to amuse her mother).

You may change your mind and decide you do want to change some mechanics, such as the damage type. If you're worried about basically copying an existing spell, then you're probably overthinking it.

Sure, it's true that some damage types are more resisted than others across all monsters in the Monster Manual. Changing a Poison spell to Force can have a big impact in some games.

Fun fact: that's totally irrelevant.

You're running a one-shot, and creating the characters for it as well. This means you have a huge amount of knowledge and control over this situation. You know that if you're setting up the party to fight a bunch of Gnolls and Goblins, the damage type doesn't matter because they have no resistances/immunities/vulnerabilities. On the other hand, if you're setting them up to fight a few Devils, then most of the baddies are immune to Poison. Therefore changing a Poison spell to Force damage would have a large impact on that spell's effectiveness in your game. Fortunately, you know all this in advance and can make an informed decision on it.

So long as you don't change actual mechanics, such as the number of enemies targeted, the range, damage type, etc, any cosmetic change you make will be fine. Even small changes to spell mechanics are not likely to break anything.