Politically, Biden's dropping out creates all sorts of uncertainty.
Procedurally, however, what happens how is extremely clear: The DNC will use its convention to officially designate its nominees for president and vice president, and those people will appear on the ballot in all 50 states, DC, and territories.
An authoritative summary of state ballot access laws can be found at https://www.nass.org/sites/default/files/reports/summary-ballot-access-laws-president-june-2024.pdf
It would have been much more complicated if this had occurred after the convention.