My term for it is "mathematical compatibility". Mechanically, it may or may not work, but the underlying mathematics is pretty consistent.
I've just written an article showing one of the unexpected incompatibilities I've encountered using a 2014 adventure with the 2024 core rulebooks.
There's a couple of interactions that may break more than you expect.
Cheers!
I think the reality is, it's up to us GMs to work with our groups to work through things like this and that's probably where it should be anyway. D&D 2024 alone has a bunch of problems home groups probably want to figure out before they hurt their game:
- Custom backgrounds instead of the goofy background selection system in D&D 2024.
- No wide open magic item crafting system.
- A bunch of magic items you probably want to be very careful with like enspelled weapons, viscious weapons, oil of sharpness, and rings of resistance.
- A bunch of spells and spell interactions you want to watch out for like upcasting summon minor elementals.
So it's in our hands now to fix our own games. Together we GMs can share our experiences and offer suggestions. We had to do this already with weird stuff in D&D 2014 ("please don't summon eight wolves and spend an hour moving them all around") and later books ("how about no twilight cleric or peace cleric" and "Just because Silvery Barbs is on D&D Beyond doesn't mean you can choose it for our campaign").
We need to take our RPG in our hands and quit assuming WOTC's going to magically fix all our stuff. I was pretty sure, when we first heard about D&D 2024, that it would fix a bunch of stuff and likely break a bunch of other stuff. For my own take, I was right. D&D 2024 may be a net positive overall (the jury is still out for me) but that doesn't mean we won't have a bunch of stuff to fix.
That's ok. We love fixing things.