What's the difference? Spells that stay in play.
I don't think you're reading it correctly.
2014:
Radiant Soul, XGE p54
Starting at 6th level, your link to the Celestial allows you to serve as a conduit for radiant energy. You have resistance to radiant damage, and when you cast a spell that deals radiant or fire damage, you can add your Charisma modifier to one radiant or fire damage roll of that spell against one of its targets.
vs.
2024:
Level 6: Radiant Soul
Your link to your patron allows you to serve as a conduit for radiant energy. You have Resistance to Radiant damage. Once per turn, when a spell you cast deals Radiant or Fire damage, you can add your Charisma modifier to that spell's damage against one of the spell's targets.
If anything, it is actually weaker... The limitation of once per turn means if you cast two spells on your turn which deal Radiant or Fire Damage, you only get to add the Charisma modifier once to
ONE spell, not both. Granted, you probably won't be casting two spells in one turn often, but it can happen. In the 2014 version, you would add your extra damage to
BOTH spells because it lacks the once per turn qualifying clause.
You only get to add the damage "when a spell you cast deals damage", on later rounds, you are not casting it on that turn any longer.
Let's not do this tangled web of present tense vs. past tense for
cast again... The use of cast is obviously in the present tense, not the past tense. Damage is dealt in the present moment in the game. If the spell was meant to work as you are reading it, they would have specified is as such.
There was nothing preventing them sticking with the old wording as they did with so much. I can therefore only conclude that the change was intentional.
Correct, they changed it to prevent it from allowing it to work on multiple spells cast on the same turn.
Not because it allows you to add it to on going damage on later rounds after the spell has been cast. It only works on the turn when the spell is cast.
There's no difference between did cast vs have cast. And because both the present and past tense of cast is cast, guess we'll never know!
There is a difference though. You cannot use the past tense here. Damage is applied in the present and it would be a disagreement of tenses. In order to do as the OP intends, it would have to be worded differently.
It can also be read as a nerf.
Before you could use it on more than one spell. Now it is only one spell per turn that can benefit.
Which is the correct interpretation of the change.
All that being the case, if a group wanted to allow the CHA extra damage to be applied on later turns of concentration spells or whatever, it certainly wouldn't be a big deal to allow it. Considering all the other changes in 2024, it would seem a minor power bump.