Honestly? We don't know. 2e started out as clarification to 1e. You can pick up a 1e module and play it pretty easily in 2e. And you could pick up a BECMI module and play it in either 1e and 2e. But 2e wound up being a major change to the game, and being quite divisive in its time.
3.0, 3.5, PF1...