The DNC is planning of formally nominating the democratic ticket on August 5, which does not leave much time to pick a replacement (even if the replacement is vice president Harris).
The August 5 date was picked back when everyone thought the nomination was a mere formality, in order to comply with an August 7 deadline from Ohio. Ohio, for its part, has pushed the deadline back to September, but I don’t think the DNC wants to risk a ballot challenge making it’s way to the Supreme Court.