I think it will be after the first debate. They're going to let him get slaughtered, which will give them the cover to replace him or Biden to step down saying he tried, but can't handle the rigors of politics anymore. And then they'll move Kamala up to the top of the ticket.
They can't replace him before the nomination with Bernie in second place in delegates because Bernie Bros will riot again, not that they still won't anyway. I think the DNC and their cronies would rather a President Harris than a President Demmings because they know Kamala is truly one of them. So I still think she will be the original VP.
To every who thinks Biden will indeed be replaced, who do y'all think they will make the new running mate? Whether it's Harris or Demmings or Abrams or even Duckworth, I have no clue who they would select as VP. I think it would almost certainly be a white male to balance the ticket but I'm not sure who.
Harris/Buttigieg would be a disastrous train wreck of a ticket. But it would set up a Pence vs Buttigieg debate which would be must see TV. Buttigieg claiming Pence hates him for being gay and Pence saying funny how I never heard this complaint when you were Mayor and I was governor.