Originally Posted by ATOBTTR

Technically what you are looking to do is against the Contract of Carriage under the "Duplicative Bookings" definition:
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DL's position is that if you end up realizing you need to stay an extra day, you make the change at the time you know and incur the fare difference. This is why airline pricing models are the way they are. Additionally, DL's position would be that you are holding inventory from someone else being able to use on the flight you end up not flying. DL's position is not technically wrong here and you canceling the flight you don't need will return the physical seat but not necessarily the fare bucket back to the inventory pool.​​​​