Select the products that you wish to enable the Pre-Order functionality:
Click on ON at the Pre-Order field (selecting OFF will bulk disable the Pre-Order):
Here's an example how it will look like:
This will enable Pre-Order for all the selected products.
If you wish to enable Pre-Order for ALL products, tick on Select All checkbox
and proceed to select the Select all <NUMBER>+ items from your store button to select the products for the whole store. If you have a huge store with thousands of items, it will take 20 to 50 seconds for it to load:
Select the ON button to enable this process:
A pop-up notification notifying you the total number of products/variants that will be affected will be shown and you are required to confirm this process by selecting Enable Pre-order button.
A progress bar will then show the progress of this process: