First of all the party doesn't have to do anything. They literally make the rules of their nominating convention. So the idea that it would just be unfettered chaos is ridiculous.

With that idea firmly in place, yes it's physically possible to replace him, until the nominating convention nominates him.

As to the idea that it must be an open convention instead of some kind of brokered convention, the earlier the democrats get on this and the more buy in they have from Biden, the more successful it will be. If they aren't going to pull the trigger though you'll never hear about anything because it could damage the campaign. So it's a crossing the Rubicon moment the second they announce they're going to do something other than nominate Biden.

first of all, some names that have been thrown around-

Newsom, high profile governor from taking runs at MAGA in the media. Was already raising his name recognition for a 2028 run. Whitmer, governor of a key state, bullet bump from the kidnap attempt Pritzker, governor of a key state Shapiro, governor of a key state

Obviously some other people are getting their names thrown around, but the party is not going to take any extra risk in a time like this. So the replacement would very likely be straight, white, male, and photogenic. I wish we lived in a world where that wasn't true, but they're trying to reach for every single vote they can and they cannot afford racism or misogyny to pull anything.

Bernie, I love him but we're not exactly going to say Biden is too old and then nominate Bernie.

How the party does this would also have an effect. If they make it an inclusive process, debates with a caucus or comment period before allowing only the top two or three (according to polls and comments) to go into the convention then they can avoid a lot of the anger over appearing to just throw Biden away. The elephant in the room here is they did not have an actual primary this year. One man's name on the ballot is not an election. So they'd need to have that sort of atmosphere, but in just the few weeks before the convention. If they don't try to include their voters then this would fail.

Over all there's a lot to be said for sticking with the guy versus taking a path with so many failure points. If they approach a candidate and that candidate leaks the plan, they're worse off than before. If they don't make the voters feel included they fail. If they allow the convention to become a chaotic mess they fail. If they can't get Biden to endorse the replacement they fail. If they choose wrong with the abbreviated vetting period and we get another October surprise then they fail. And there's always the chance that they sustain too much damage even though they do everything right and fail anyways.

But yeah, it's possible, it doesn't have to be chaos, and there are people who can step in.