If another person becomes the nominee, the Biden campaign could refund contributions so they can be sent to the new campaign directly.
Hmm.
That’d have to happen extremely quickly. If they don’t have some kind of mechanism already in place for getting approval from the donor, it seems likely to me that they wouldn’t have time to set something up.
The US typically runs fairly long campaigns, the whole election year. Not all countries work like that. IIRC, the UK does a (limited) three month campaign cycle. But even by those standards, this is really short. There are about three-and-a-half months left before the election. They haven’t even selected an alternative, much less had someone spend the money to put together a campaign, much less actually embark on it.
Also, US campaigns are very large compared to most countries. I don’t know what total spending is like this time around, but I remember that when Trump ran against Hillary in 2016, each spent about $1 billion in their campaign. If you have to do that, you’d have to select someone, set up and complete all the fund transfer stuff, pay someone to devise a campaign, and then implement the campaign – and this is on the order of a billion-dollar project – in about a hundred days.