Ford changed the Bronco FAQ on their website. You now have to cancel your existing reservation and create a new one with the dealer you're switching too.
Here's what the website now says:
Can customers update their dealer selection for their reservation?
To change dealers after their...
I can’t speak on moving an order but I can tell you once the order is placed you can’t make any changes on the order but must cancel order and have a new one issued which puts you further back in line, because it’s a new order, You may want to check to be sure you keep your original reservation...