"Biden For America" or whatever, is going to hand that money over to the person they pick?
They can't do this. Election law stipulates that money raised by one candidate can't be given to another. The new candidate would have to start from scratch.
There's also the problem of state laws concerning the ballot. Some states have a deadline after which no changes can be made. I believe Georgia's is July 7th. I think that's the earliest, but I could be wrong.
"Biden For America" can't just give money to "Whitmer for America" (or whatever) or to the DNC but there are multiple things they CAN do with it, some of which could be very helpful to candidate Whitmer. Probably what they would do is form a new Super Pac with the money dedicated to helping their allies win races. Allies who might be grateful to have checks written for them.
They CAN give the money to 'Kamala Harris for America' since she's on the current ticket. But the Bidens would then have no (zero) authority to say what was done with the money.
The point is that the Bidens aren't going to give away control of that money. The only candidate who can have control (or deep coordination with) the Biden For America money is Harris, if they give the warchest to her.