Add a new delivery date (update pre-order info) after reaching the pre-order limitation

I have already ordered 100 items at my wholesaler that will be delivered on time (so that the customers receive their order on March 1st).

When the 100 items have been sold, customers should still be able to order the product, but they won't receive it on March 1st since I'd have to place a backorder at the wholesaler. So they can still order, but it'll be delivered as soon as I receive their product from the wholesaler.