Here are the current DNC Convention rules: https://democrats.org/wp-content/uploads/2023/03/2024-Call-for-Convention.pdf
IX. Procedural Rules of the 2024 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.
Per wiki, In 2021, Jaime Harrison was selected by President Joe Biden to Chair the Democratic National Committee, and his nomination was approved by its members.
The Current Leadership is:
Chair: Jaime Harrison, former Chairman of the South Carolina Democratic Party
Executive Director: Sam Cornale, former Chief of Staff of the Democratic National Committee
Deputy Executive Director: Roger Lau, former Campaign Manager for Elizabeth Warren for President
Political Director: Alana Mounce, former Executive Director of the Nevada State Democratic Party
Chief of Staff: Anatole Jenkins, former National Organizing Director for Kamala Harris For The People
Vice Chair of Civic Engagement and Voter Participation: Keisha Lance Bottoms
Vice Chairs:
Gretchen Whitmer, Governor of Michigan
Tammy Duckworth, U.S. Senator from Illinois
Ken Martin, Chair of the Minnesota Democratic-Farmer-Labor Party
Treasurer: Virginia McGregor
Secretary: Jason Rae
Finance Chair: Chris Korge
In addition, a National Advisory Board exists for purposes of fundraising and advising the executive. The present chair is Elizabeth Frawley Bagley, former U.S. Ambassador to Portugal.
Interesting. I guess it’s not surprising that they’d reserve as much discretion for themselves as possible, but their hands may be tied. Once Biden/Harris is nominated, the states will print their ballots and it will be very difficult to get any substitute onto the ballot in many states. Their best bet will be to tell voters to vote for Biden/Harris and since Joe won’t be sworn in even if he wins, Harris will succeed to the Presidency automatically under the 25th amendment.
On the other hand I would not be surprised to see Dem chicanery if the Biden/Harris ticket won. They might try to get their electoral college electors to vote for their chosen substitute instead of either Biden or Harris. And if people complain about that, they’d say that it means the electoral college should be abolished.