Faolyn
(she/her)
We already know the classes are different, since that's a major point of the game.Absolutely right, which is why it is compatible with "adventures" despite being a different system. It is not compatible with D&D classes, or D&D rules, because it is a different rule system that overwrites D&D.
But I'd think you'd have to go through an adventure and find the places where the games are actually incompatible to the point that an A5e character couldn't play in it without making huge changes.
Like, you mention Press the Attack/Fall Back--but that's just a combat action. That's not going to affect the adventure, just the combat. You could let an o5e PC use that without problem, just like you could let them use Sprint.