I read in some classes sub-discipline that they can use certain spells as any dmg type. Like cast a fireball as lightning damage. Now I can't remember which it was.

Anyone remember?

Transmuted Spell is a meta magic option Sorcerer's can take that appeared in Tasha's Cauldron of Everything.  As written it's limited to transmuting between acid, cold, fire, lightning, poison and thunder. 1 Sorcery point/use.

As Midnight said, there's a Metamagic option for Sorcerers. There is also Order of Scribes Wizard which at 2nd level has a feature in the Awakened Spellbook that lets you replace the damage type of a spell with the damage type of another spell within your book.

Edit:

It also says the spell you're using for the damage type replacement needs to be of the same level as the spell you're casting.

That's also a benefit of the Order of the Scribe wizard's Awakened Spellbook, though there's a minor limitation - the new damage type has to appear in another spell in the spellbook.

(Somewhat telling that sorcerers are expected to use one of their few metamagic choices and a sorcery point per use for this, while it's only a 2nd level subclass feature (and not even all of it) for a wizard - not that I'm complaining or anything :p)

Dang, I didn't know that about the scribe, but yeah ... smh.  I was actually surprised the transmutation thing didn't show up in 5e until Tasha's because it's the first thing that popped into my head when I first heard about metamagic.  But I guess Wizards can reduce everything into an academic arcane exercise so....

It also says the spell you're using for the damage type replacement needs to be of the same level as the spell you're casting.

Having played Order of the Scribe up to level 8 so far, this restriction is actually pretty huge unless your DM is just dumping extra spells on you left and right.

Many of the best wizard spells do no damage at all. If you really want blasting versatility you need to mostly pick blasting spells, but that cuts into the utility and control that IMO is the strongest aspect of wizards. What most will end up with is a few damage options per spell level, with lower level spells seeing more use because you can upcast them to the level that has the element you need. It's still pretty strong, but it requires a lot of tradeoffs and strategy - the metamagic ability to just do it to any spell at any time is way more flexible and probably should be more expensive.

Quote from scatterbraind >>

It also says the spell you're using for the damage type replacement needs to be of the same level as the spell you're casting.

Having played Order of the Scribe up to level 8 so far, this restriction is actually pretty huge unless your DM is just dumping extra spells on you left and right.

Many of the best wizard spells do no damage at all. If you really want blasting versatility you need to mostly pick blasting spells, but that cuts into the utility and control that IMO is the strongest aspect of wizards. What most will end up with is a few damage options per spell level, with lower level spells seeing more use because you can upcast them to the level that has the element you need. It's still pretty strong, but it requires a lot of tradeoffs and strategy - the metamagic ability to just do it to any spell at any time is way more flexible and probably should be more expensive.

It's huge if you're hoping to target vulnerabilities. If you're just looking to avoid resistances and immunities, it's not that bad. Sorcerers pay through the nose for metamagic (if you consider untility and control to be the strongest aspect of wizards you're arguably correct - and it's an absolutely massive advantage they have over sorcerers), they only get 4 types of it (more realistically 3, since getting to level 17 is exceedingly rare), and it's pretty debatable whether Transmuted Spell is among the best choices. 

Thank you everyone for answering my question and more :)

Ain’t there the Chronurgy subclass too, or am i being stupid?

Quote from SpeedyTabaxi_CatsForLife >>

Ain’t there the Chronurgy subclass too, or am i being stupid?

 Neither Chronurgy school allows you to change the elemental type of a spell.

Yeah, i thought of Lore wizard

Quote from scatterbraind >>

It also says the spell you're using for the damage type replacement needs to be of the same level as the spell you're casting.

Having played Order of the Scribe up to level 8 so far, this restriction is actually pretty huge unless your DM is just dumping extra spells on you left and right.

Many of the best wizard spells do no damage at all. If you really want blasting versatility you need to mostly pick blasting spells, but that cuts into the utility and control that IMO is the strongest aspect of wizards. What most will end up with is a few damage options per spell level, with lower level spells seeing more use because you can upcast them to the level that has the element you need. It's still pretty strong, but it requires a lot of tradeoffs and strategy - the metamagic ability to just do it to any spell at any time is way more flexible and probably should be more expensive.

 Yeah whenever I play a wizard I always want to have that conversation with the DM going into it, especially with the Scribe wizard that gets such a boost when it comes to transcribing spells. You want there to be the opportunity to use that and increase your spell list, because 2 per level isn't really going to cut it for more than a couple situations.