Originally Posted by Paplover

In other words, without SDC you are cancelling your original ticket and buying a new ticket, including the return if on an outbound. As mentioned, even if the flights are empty, you must buy a fare class with the new purchase date and even if cheap fares are “available” you likely will not meet advanced purchase requirements and cannot buy them. If you started with pricey fares it might be free/cheap to buy the new ticket, other times expensive. SDC lets you change a flight without replacing and repricing the whole ticket. This is especially important if other segments on your ticket are not available at low fare classes or sold out. DL charges $75 for SDC unless you are GM or higher and SDC avoids impacts on the rest of your trip.