What does it mean that backwards compatibility in schema evolution?

Sometimes you need the new schema to handle cases the old schema covered.

Example: You upgrade your worldview from 'hard work always leads to success' to a more nuanced model that accounts for luck, privilege, and timing. But if your new schema can't explain why your friend who worked 80-hour weeks did become successful, you haven't upgraded — you've just swapped one incomplete model for another. Your evolved schema must handle every case the old one handled, plus the cases that broke it.

Try this: Identify one belief you've recently updated. Write down three situations where your old belief gave you a correct prediction. Now test: does your new belief also give correct predictions for those same situations? If not, your new schema isn't backwards compatible — it's just different, not better. Revise it until it covers both the old successes and the new cases that forced the change.