Or people could actually go out and tell others to vote.
Sure. This has been the strategy for a while, and has resulted in likely Biden voters becoming less engaged due to genocide and the general percieved cognitive decline as of late, and Trump voters becoming more engaged with events like the failed assassination attempt.
Biden voters are largely going to end up holding their nose, while Trump voterd are largely going to gleefully show up in droves, if conditions continue.
I don't think this is a useful strategy, and will be a repeat of 2016.
I don't see just changing candidates doing anything; in fact, you'd probably get a 'where were all the primaries?' backlash
If the majority of people support swapping out Biden, I think you'll find that it won't result in fewer voters. The DNC could also organize an emergency Primary, too. Doing nothing is handing Trump the win on a silver platter.
Approximately 100% of Biden voters would vote for his replacement. A non-zero amount of voters would also vote for this person. At this point sticking with Biden is conceding the election.
While I don't think Biden should have run for a second term, that's not the scenario we live in. He is running, no viable Dems ran against him in the primaries. He will be their nomination. Anything else is almost Fantasyland.