Subbing a candidate out for Biden-- possible?

Could someone with real political knowledge help me out on this-- aren't there deadlines by state right around November/December of this year that you have to file for in order to be on the ballot next year? And those filings are very public record and there are no doubt thousands of people scouring these daily?

Is the above true? If true, wouldn't we know if they're planning a switch? Is there a little known loophole Democrats can take advantage of to get a new candidate on the ballot sometime next year?

ETA:

I did some research instead of being lazy:

There is only a deadline to be on state ballots, not to be a nominee. You can be a nominee at any point.

On the Democrat side, delegates become "pledged" based on primary elections and are required to vote for the candidate based on their state's primary if they're still in the race.

If a candidate with pledged delegates drops out during the race after a state's primary, their delegates become "unpledged" and they can vote for whichever candidate they choose absent any democratic process. Since Biden will get practically all the delegates, they can choose whoever they want at the convention.

This is how they would pull off a candidate switch.