There is already a question
There is already a question of whether Tesla Model 3 buyers will be eligible for the federal tax credit. Converting those Model 3 sales into Model S sales would make it more likely that Model 3 buyers will be out of luck for tax credits.
Besides, that "backlog" doesn't actually exist. There aren't any orders for the Model 3. There aren't even preorders. There are *reservations* which can turn into orders once they actually have a car to sell.
So, this is a bad solution for a problem that doesn't exist.