A couple things: this only applies to the Biden/Harris Presidential Campaign account – not to funding held by the DNC or Biden-aligned PACs, who are free to redirect their spending however they like.
The informed commentary I’ve seen opines that Harris would be able to take over the Presidential campaign account because her name is as much on it as Biden’s. But it is unprecedented, and I’m sure it’ll end up in court. If for any reason she couldn’t, Biden’s campaign could refund the money to donors with the request that they contribute to her campaign or transfer an unlimited amount to the DNC, which could then spend the money supporting Harris.