Technically what you are looking to do is against the Contract of Carriage under the "Duplicative Bookings" definition:
Duplicate Bookings
All duplicate bookings generated by a single GDS subscriber are prohibited, including:
Multiple itineraries for any number of passengers with the same passenger name, whether identical itineraries or not
Reserving one or more seats on the same flight or different flights for the same time frame, regardless of the class of service or format used to make the reservations
https://www.delta.com/us/en/legal/bo...cy-definitions
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.

Is DL going to catch you in a one-off and in a scenario where you could with a degree of some reasonableness say you intend to take both flights if questioned? Probably not. That said, if your company needs you to change flights because of work, they should cover the costs anyway of the flight change if there is a fare difference and hey, that's more MQDs and Miles for you anyway. Otherwise you could also just book the "back-up" segment on another airline that operates LA-SF and there's no way for either airline to know you're engaging in duplicate bookings because you aren't engaging in duplicate bookings with a single airline (you are still holding inventory someone else could utilize).

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