If they switch any later than mid-October, early voting will destroy the Democrats:
https://www.ncsl.org/research/elections-and-campaigns/early-voting-in-state-elections.aspx
Democratic votes will already have been cast for Joe Biden, and they will not be able to put that toothpaste back in the tube.
The rats know that too so the switch will be in Sept just before early voting is opening up in the states that matter to them. That accounts for the delay in the announcement we just learned about.
It may be after the ballots are printed but the vote could be write ins in most states. I can even see the rats claiming that a vote for a disqualified biden should count for their new guy. That'll fire up the lawsuits for quite some time.
History shows that people initially get excited for the "new guys" before their dirty laundry gets out. That bump can make the difference in a 4 point race.
On the plus side, the bernyites will be really pissed it isn't their guy and they can vote accordingly too. I see a strong attempt for them to do the ron paul thing "rats...vote you conscience".