No, if Biden resigns, dies, or is disabled, the VP, Harris, would finish out the term. Then the November elections happen. The DNC would need to figure out who to run in November.

In this scenario, Harris, would NOT necessarily be the presumptive nominee for running in November.

If Biden is being replaced as the nominee, I think that stepping aside before the DNC convention would be less chaotic.

The clock is ticking. There is a ground swell of support for Trump that a new DNC candidate would need to surmount. Whoever the DNC runs would need to be nationally recognized (Hillary, Harris, M. Obama) or this becomes another factor that needs to be surmounted. Newsome, Whitmar have regional name recognition. They are nationally recognized among political hacks like us, but regular folks, not so much. The DNC is quickly running out of runway to enact changes.

I remember the MN senate race between Coleman and Wellstone. Wellstone's plane goes down a week or two before the election. Wellstone was killed. They had to drag Walter Mondale out of mothballs to run because they needed the instant name recognition. They were out of time.