The one we ran into was elegance (I think) that lets you choose to spend the die and if you fail you can keep the die to use it again. However the new system is not that you have the ability to spend it.
So the confusion was who “keeps” it the bard or the target.
Respectfully, I think you’ve overthought this one. The text of the level 6 feature of the Eloquence Bard says that “When a creature
adds one of your Bardic Inspiration dice to its ability check, attack roll, or saving throw and the roll fails,
the creature can keep the Bardic Inspiration die.”
The creature keeps it. That is totally unchanged by changing the BI mechanic to a reaction. When they add your die to a roll and it fails, they keep the die. That’s it.
2014 bard, the creature just uses it when they roll, 2024 probably uses it as a reaction, which is a slight nerf, but that’s it.
I think there is one (it might be 3rd party have to look) that when you use your bardic inspiration choose a second target but that target won’t be useing something right then and there.
I’d have to see it to comment fully, but if the reaction BI feature makes it to print, it doesn’t seem like a big deal to include half a sentence clarifying how to deal with this sort of thing.
Edit: in light of moderation
There isn’t much point in discussing things at all if we just assume they will fix everything anyway.
Discussing potential issues is distinct from harping on every last little potential sticky wicket until other people are getting tired of even discussing any of it as a result.
So…it’s compatible. It works with both versions.
The only thing that doesn’t work is trying to create a Frankenstein class by cherry picking base class features between the two.
If you choose the 2014 warlock, you use that warlock. You use its spell list, Spellcasting table, you use its invocations, its versions of the pact boons, etc.
If you choose the revised warlock, you use the PHB (2024) spell access, invocations, Spellcasting table, proficiencies, etc.
The only way there could be an incompatibility is if the 2014 subclasses can’t work in the revised class, or the reverse. Since we know that they’re gonna try a few different Spellcasting models for the class, I’m just not seeing compatibility as a useful topic of discussion yet, in this case. What matters is where the class as written
works. Compatibility can be solved when a model has been chosen.