If its the same dealer group you have ordered from there shouldn’t be much of an issue. If its a completely different dealer you may run into issues, as it would essentially trigger a new order, which isn’t an option.
The only way I can see around this, is asking your current dealer who you have ordered the car from, to physically allow the transfer of the order across to the new dealer, hence retaining the same order number or asking your dealer if they would be willing purchase the car from the dealer who has one in stock, however this may involve transport costs.
Good Idea on the surface but, may prove difficult in reality 🙂