I would have thought so but I think the only way is for Biden to be persuaded to stand down. And I doubt any new candidate will be able to deal with the charge that the Democrats were putting an incompetent candidate forward who is also the President.
I think the Dems wanted a debate this early in the electoral cycle (well before the conventions) to allow them to pivot to a new candidate if this happened in the early debate. Remember the Dem convention isn't until mid/late August so there is plenty of time to tee up a successor.
The spin would, of course, be that his health had deteriorated in a manner than they hadn't expected. The problem will be to argue that this means they can't really put him forward as a credible candidate for the next 4 years, but he's still OK as president now.
First - there is no certainty the Dems will actually do this - after last night it is more likely but I'd still say it remains more likely than not that Biden will remain their candidate, unless he, himself, recognises that he can't be.
Secondly - how would a change in candidate affect overall public opinion. Well that's very hard to say and it would depend on who that new candidate is. However one of the main narratives around the election so far is that the public really don't want Biden vs Trump again, so potentially ANOther vs Trump may be seen as a really positive move.
