What are the ramifications of a caster changing the ability of a forced saving throw?

Question

There was an Unearthed Arcana release including a Wizard subclass called Lore Mastery. The development of this subclass was largely abandoned, although some of its features inspired a later UA subclass, the School of Invention.

One of the features abandoned with the Lore Master, the 4th-level Spell Secrets feature, allowed the caster to change the ability of a saving throw forced by a spell, once per short or long rest:

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.

I've permitted a player to use the Lore Master, including the save changing feature, in a campaign despite the abandonment of the subclass, since we both really like the concept. We've rarely seen the feature put to use, although in those few uses it has been interesting, helpful, and free of issues for us. However...

What are the ramifications (in terms of balance implications, exploits, and problematic rules interactions) of a feature that allows a caster to change the ability for which a spell forces a saving throw? Assume the feature has a limited use, such as once per short or long rest.

Despite the background I've included, I'm interested in a general answer for any spellcaster, not just a Lore Mastery Wizard using that specific feature.

Answer 1

Exploits

Have a look at Hold Person: WIS save or paralyzed for 1min, can repeat the WIS save at the end of turn. Should the save be changed to STR or DEX, the creature will automatically fail the save and it's basically game over.

Note that ruling that they are separate saves (the initial and the continuous) isn't really helping, as this way you can keep the initial WIS save (a lot of creatures have higher STR than WIS anyway). Probably the only way to balance it is to rule that only the initial save can be changed but that's a bit weird flavour-wise.

And that's just one spell; balancing it, as you ask, needs to go through all features/spells/etc and consider them assuming that any save can change (possibly the reason the concept was abandoned). Some people have voted that the question is too broad; but I think that that's the actual answer: there are far too many potential interactions to examine to ensure the feature is balanced.

Kinda useless

On the other hand, the feature is quite weak; it can only be efficient if you know the weak save of a monster. That wouldn't actually be surprising for a character that focuses on lore and has high INT. But if you don't provide this information, then they'll probably default to INT saves, same as the elemental damage swap will almost always be force.

Fundamentally, the feature is too swingy: at its best, it creates a single save-or-die spell; at its worse it makes a save slightly harder (or even easier, if the player doesn't know the target well). Maybe limiting it to spells that don't require saves to be repeated and not accepting a swap to INT without a very, very good explanation (no "it's really an illusion" excuse), would make it weak enough to give it for free or, bundled with the damage type swaps (excluding force), as a feat. It seems a bit more powerful than elemental adept, given that few creatures have vulnerabilities but it'll require some play-testing and vary from table to table.