I think that was pretty much the entire point of scheduling a debate this early - before the Dem convention even occurred.

From a a game-theory view, there was no clear upside for Biden to participate in a debate this early....he is the incumbent President, its assumed he will be running. If he did an outstanding job, he would just remain the nominee, if he faltered he would be under pressure to be removed. He only stood to lose.

I don't think the "we can just swap out Biden" theory is going to go quite as smooth as Dems hope though....the 'Lügenpresse' has spent the last 4 years lecturing the public that Biden is doing a wonderful job, its going to come across as quite desperate at this sage to suddenly have him thrown under the bus and try to frantically build support behind another candidate.