It wouldn't be a problem problem, but it might annoy them enough that they decide not to bother trying another candidate. And what would the parliamentarian say?!? NYT:

But there is a catch, one that has been informed by recent precedent. If someone other than Ms. Harris were to lead the ticket, the Biden-Harris campaign could certainly transfer that $96 million to a federal super PAC, which could spend it on advertising for the new Democratic ticket.

That would be less than ideal, because money at an outside group cannot be controlled directly by the campaign, and super PACs have to pay higher rates for advertising time. But the money would not go to total waste.