Right now, it's just a reframing of the various character build elements with some slightly different philosophies used. The change is about at the level of a base 4e->Essentials to me. Now, if they do something like make pre-2024 subclasses not work with 2024 classes, or change the skill list, then we're at a level of change where I feel like "backwards compatible" isn't credible.
And honestly, 2e really didn't need to be called a different edition, anyway. I'm not an expert on 1e, but I recall very little changing other than level limits being raised and NWP being made standard. Was there anything that stopped a 1e half-orc assassin from being playable in a 2e game?