Getting rid of Joe will be easy. They'll get him to step down for health reasons. First, however, they need to remove Kamala from the VP spot, which will be much trickier. My theory is they'll create a new cabinet post ("Secretary of Infrastructure?"), and move Kamala into that job. They'll sell it as a "lateral move." Kamala will be paid a boatload of money behind the scenes in exchange for agreeing to "enthusiastically accept this important and challenging assignment." Kamala gets to save face, and they'll then move their preferred 2024 nominee into the vice presidential spot.
That would leave the POTUS position empty.
Well, if Joe just decides not to run again, then the Kamala problem is automatically solved. Joe only gets the courtesy of an unopposed nomination if he actually runs for reelection. If he doesn’t, then the field is open and Kamala isn’t guaranteed anything at all.