I don't know who the replacement would be, but the person at the top of the ticket names their running mate. Harris doesn't have to be asked to step aside.
My guess would be a governor who isn't a white male, who's from a swing state. Perhaps Whitmer.
Asking her to step aside is the best case scenario. In that scenario she announces that she is voluntarily withdrawing, but everyone knows what's really happening. Naming anyone else without that is just an outright betrayal.