1. Open the Booking's "Inventory" tab.
2. If the Booking currently has some inventory allocated, you will first need to click 'Clear Booked Inventory' at the bottom of the page.
3. After clearing the inventory, you will have a button to change Product. If you just wish to switch to a different Product Option (Package) then click the "Add Inventory Allocation" button and set the new Product Option in the dialog that appears.
In this case, it might be preferable to do the following:
1) copy the booking to create a new booking against the new product. The new booking will include a link back to the original booking.