If you are talking about the 2024 announcement the wording wasn't "compatible", it was "fully compatible".
I read that to mean that you can 100% create any existing character. No grandfathered characters will need a single change. So changes like recalibrating the encounters-per-day, as much as I would dreadfully love such a thing, can not be on the table.
I think we'll get exactly what they have shown us with Tasha's. Races with ability scores assignable, and possibly changing the name from "race".
Classes with purely optional (so not required on existing characters) new features, like existing sorcerer subclasses getting free spells. Classes with more choices for features, allowing you to replace problematic ones as well as just give new options. Both of these were in Tasha's as well.
In other words, I read "fully compatible" as "you can create the exact same character as before and run it with other characters with no more problem then right now (which might be a level of problem), but there will be new options that might make you want to create it a different way".