resign the presidency or died at the end of this month, for example, you just might get in enough under the wire for primaries to pick his replacement, with candidates working against a now incumbent President Harris. In such a short timeline, I don't see that happening though, so it's possible that a brokered convention could pick a different candidate, again against President Harris with now four or five months under her belt. Unless she were a total fuck up, I don't see that happening either.

If he resigned or died after the convention, same question but in spades. You have an even shorter timeline, and it's likely that President Harris then picks and the DNC confirms, a new Veep candidate for November. I would not see the DNC replacing her.

If he just decided not to run again but not step aside, say at the end of this month, then you have a chance to start with a clean slate with primaries. Biden would then select who his existing delegates, he has them all, go to. In that case, Harris may or may not choose to run. My sense is that she might not and she might not win if she did run in any case. She stands a far, far better chance if she's already President.