Then what is the major difference after emancipation?

Exactly what you said: the legal basis of slavery was gone.

That is the fundamental transformation.

Before:

A person could legally be owned.

After:

A person could no longer legally be owned.

That is not a minor change.

It is a fundamental legal transformation.

Your argument should preserve that point very strongly.

You are not saying:

“Nothing changed.”

You are saying:

“A fundamental legal transformation occurred, but it did not automatically transform every social and economic relationship surrounding the formerly enslaved.”

That is much stronger historically.