Before the convention, the rules follow the primary and votes. Once Biden is official Asher the convention entirely different rules kick in. The DNC chair consults with the den governors and then chooses someone.
They could very easily do it and snake Bernie out again.
IX. Procedural Rules of the 2020 Democratic National Convention
G. Filling a Vacancy on the National Ticket: In the event of death, resignation or disability of a nominee of the Party for President or Vice President after the adjournment of the National Convention, the National Chairperson of the Democratic National Committee shall confer with the Democratic leadership of the United States Congress and the Democratic Governors Association and shall report to the Democratic National Committee, which is authorized to fill the vacancy or vacancies.