So I'm still not 100% sure on requirements for such things but a few friends and I were talking and thought if we were Democrat strategists and wanted to do a quick reset with the least amount of damage we'd get rid of Biden, like yesterday. If you can shut him down on his own great but if you have to 25th him then do it. Install Kamala ASAP and get her out in front of as many people as possible and make it rough on her. Imagine the MSM going at her like she was a right wing Republican right up to the convention. If she somehow exceeds all expectations and steps up to the point she's looking like a genuinely viable candidate and polling well against Trump then so be it. Run with her. Given that seems exceedingly unlikely and she tanks you don't have to play the "We just straight decided to go around the female POC for POTUS." card. The best case scenario there is she's polling, well, pretty much like she is now and steps down. Even if she doesn't it can be clearly presented that she isn't really a viable nominee. Then at the convention you have whomever (Newsome?) on deck and go from there.
It's pretty late in the game, convention starts 8/19, but the above looked like the only way to clear the slate with the least collateral damage. (or at least the best we noodled up under the influence of holiday party drinking)