Gadolhadorah
You are quite wrong about the mechanics.
If a candidate releases delegates, they may vote for others at the convention.
It is not Biden who has a place on the 50 state ballots, but the official nominee of the Democrat Party and that person is not chosen until the convention in August.
In most states absentee ballots can be mailed 30 days before Election Day, not soon after the August Convention.
Replacing a named candidate on the ballots after the convention can be tricky.
FYI, I still hold my position as Asst. Registrar of Voters back in CT and have to keep up on the laws. I will be overseeing the August 13 primaries this year.